merge: add tests for commit with no content change

It isn't easy to say when to reuse the p1 manifest. Basically, that's only
when wctx.files() is empty, but we need to know that wctx.files() is not
the same as repo['.'].files() after the commit.

This patch adds several examples of commits with empty ctx/wctx.files().
I don't think this is exhaustive, but it contains at least one failure
mode in which a converted repo result in a different hash.

I also note that the manifest revlog does NOT follow the DAG shape of the
changelog since p1 manifest is reused if wctx.files() is empty even at merge.
I don't know whether it is intentional or not, but it's the behavior since
2011, 301725c3df9a "localrepo: reuse parent manifest in commitctx if no files
have changed."

tests/test-merge-no-file-change.t

@@ -0,0 +1,386
+  $ cat <<'EOF' >> "$HGRCPATH"
+  > [extensions]
+  > convert =
+  > [templates]
+  > l = '{rev}:{node|short} p={p1rev},{p2rev} m={manifest} f={files|json}'
+  > EOF
+
+  $ check_convert_identity () {
+  >     hg convert -q "$1" "$1.converted"
+  >     hg outgoing -q -R "$1.converted" "$1"
+  >     if [ "$?" != 1 ]; then
+  >         echo '*** BUG: hash changes on convert ***'
+  >         hg log -R "$1.converted" -GTl
+  >     fi
+  > }
+
+Files added at both parents:
+
+  $ hg init added-both
+  $ cd added-both
+  $ touch a b c
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ hg ci -qAm2 c
+
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  committing files:
+  b
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 3:7aa8a293f5d97377037afc21e871e036e718d659
+  $ hg log -GTl
+  @    3:7aa8a293f5d9 p=2,1 m=3:8667461869a1 f=[]
+  |\
+  | o  2:e0ea47086fce p=0,-1 m=2:b2e5b07f9374 f=["c"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+  $ cd ..
+  $ check_convert_identity added-both
+
+Files added at both parents, but the other removed at the merge:
+(In this case, ctx.files() after the commit contains the removed file "b", but
+its manifest does not differ from p1.)
+
+  $ hg init added-both-removed-at-merge
+  $ cd added-both-removed-at-merge
+  $ touch a b c
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ hg ci -qAm2 c
+
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg rm -f b
+  $ hg ci --debug -m merge
+  committing files:
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 3:915745f3ca3d9d699925269474c2d0a9526e8dfa
+  $ hg log -GTl
+  @    3:915745f3ca3d p=2,1 m=3:8e9cf3456921 f=["b"]
+  |\
+  | o  2:e0ea47086fce p=0,-1 m=2:b2e5b07f9374 f=["c"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+  $ cd ..
+  $ check_convert_identity added-both
+
+An identical file added at both parents:
+
+  $ hg init added-identical
+  $ cd added-identical
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ touch b
+  $ hg ci -qAm2 b
+
+  $ hg merge
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  reusing manifest from p1 (no file change)
+  committing changelog
+  updating the branch cache
+  committed changeset 3:de26182cd210f0c3fb175ca7616704ab963d3024
+  $ hg log -GTl
+  @    3:de26182cd210 p=2,1 m=1:686dbf0aeca4 f=[]
+  |\
+  | o  2:f00991f11eca p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+  $ cd ..
+  $ check_convert_identity added-identical
+
+#if execbit
+
+An identical file added at both parents, but the flag differs. Take local:
+
+  $ hg init flag-change-take-p1
+  $ cd flag-change-take-p1
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ touch b
+  $ chmod +x b
+  $ hg ci -qAm2 b
+
+  $ hg merge
+  warning: cannot merge flags for b without common ancestor - keeping local flags
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ chmod +x b
+  $ hg ci --debug -m merge
+  committing files:
+  b
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 3:4bfaad7f925b7f17f60524dc5d4e605f7bfbba3f
+  $ hg log -GTl
+  @    3:4bfaad7f925b p=2,1 m=3:a3a9fe23a5b8 f=[]
+  |\
+  | o  2:99451f16b3f5 p=0,-1 m=2:36b69ba4b24b f=["b"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+  $ hg files -vr3
+           0   a
+           0 x b
+
+  $ cd ..
+  $ check_convert_identity flag-change-take-p1
+  3:c8d50407916e
+  *** BUG: hash changes on convert ***
+  o    3:c8d50407916e p=2,1 m=2:36b69ba4b24b f=[]
+  |\
+  | o  2:99451f16b3f5 p=0,-1 m=2:36b69ba4b24b f=["b"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+An identical file added at both parents, but the flag differs. Take other:
+
+  $ hg init flag-change-take-p2
+  $ cd flag-change-take-p2
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ touch b
+  $ chmod +x b
+  $ hg ci -qAm2 b
+
+  $ hg merge
+  warning: cannot merge flags for b without common ancestor - keeping local flags
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ chmod -x b
+  $ hg ci --debug -m merge
+  committing files:
+  b
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 3:06a62a687d87c7d8944743dee1ee9d8c66b3f6e3
+  $ hg log -GTl
+  @    3:06a62a687d87 p=2,1 m=3:2a315ba1aa45 f=["b"]
+  |\
+  | o  2:99451f16b3f5 p=0,-1 m=2:36b69ba4b24b f=["b"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+  $ hg files -vr3
+           0   a
+           0   b
+
+  $ cd ..
+  $ check_convert_identity flag-change-take-p2
+
+#endif
+
+An identical file added at both parents, one more file added at p2:
+
+  $ hg init added-some-p2
+  $ cd added-some-p2
+  $ touch a b c
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg ci -qAm2 c
+  $ hg up -q 0
+  $ touch b
+  $ hg ci -qAm3 b
+
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  committing files:
+  c
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 4:f7fbc4e4d9a8fde03ba475adad675578c8bf472d
+  $ hg log -GTl
+  @    4:f7fbc4e4d9a8 p=3,2 m=3:92acd5bfd716 f=[]
+  |\
+  | o  3:e9d9f3cc981f p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  | |
+  o |  2:93c5529a4ec7 p=1,-1 m=2:ae25a31b30b3 f=["c"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+  $ cd ..
+  $ check_convert_identity added-some-p2
+
+An identical file added at both parents, one more file added at p1:
+(In this case, p1 manifest is reused at the merge commit, which means the
+manifest DAG does not have the same shape as the changelog.)
+
+  $ hg init added-some-p1
+  $ cd added-some-p1
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ touch b c
+  $ hg ci -qAm2 b
+  $ hg ci -qAm3 c
+
+  $ hg merge
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  reusing manifest from p1 (no file change)
+  committing changelog
+  updating the branch cache
+  committed changeset 4:a9f0f589a913f5a149dc10dfbd5af726977c36c4
+  $ hg log -GTl
+  @    4:a9f0f589a913 p=3,1 m=2:ae25a31b30b3 f=[]
+  |\
+  | o  3:b8dc385241b5 p=2,-1 m=2:ae25a31b30b3 f=["c"]
+  | |
+  | o  2:f00991f11eca p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"]
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"]
+  
+
+  $ cd ..
+  $ check_convert_identity added-some-p1
+
+A file added at p2, a named branch created at p1:
+
+  $ hg init named-branch-p1
+  $ cd named-branch-p1
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg ci -qAm1 b
+  $ hg up -q 0
+  $ hg branch -q foo
+  $ hg ci -m2
+
+  $ hg merge default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  committing files:
+  b
+  committing manifest
+  committing changelog
+  updating the branch cache
+  committed changeset 3:fb97d83b02fd072295cfc2171f21b7d38509bfd7
+  $ hg log -GT'{l} branch={branch}'
+  @    3:fb97d83b02fd p=2,1 m=2:9091c64f4ea1 f=[] branch=foo
+  |\
+  | o  2:a3a9fa6587e5 p=0,-1 m=0:8515d4bfda76 f=[] branch=foo
+  | |
+  o |  1:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"] branch=default
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"] branch=default
+  
+
+  $ cd ..
+  $ check_convert_identity named-branch-p1
+
+A file added at p1, a named branch created at p2:
+(In this case, p1 manifest is reused at the merge commit, which means the
+manifest DAG does not have the same shape as the changelog.)
+
+  $ hg init named-branch-p2
+  $ cd named-branch-p2
+  $ touch a b
+  $ hg ci -qAm0 a
+  $ hg branch -q foo
+  $ hg ci -m1
+  $ hg up -q 0
+  $ hg ci -qAm1 b
+
+  $ hg merge foo
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  reusing manifest from p1 (no file change)
+  committing changelog
+  updating the branch cache
+  committed changeset 3:036823e24692218324d4af43b07ff89f8a000096
+  $ hg log -GT'{l} branch={branch}'
+  @    3:036823e24692 p=2,1 m=1:686dbf0aeca4 f=[] branch=default
+  |\
+  | o  2:64d01526d4c2 p=0,-1 m=1:686dbf0aeca4 f=["b"] branch=default
+  | |
+  o |  1:da38c8e00727 p=0,-1 m=0:8515d4bfda76 f=[] branch=foo
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"] branch=default
+  
+
+  $ cd ..
+  $ check_convert_identity named-branch-p2
+
+A file changed once at both parents, but amended to have identical content:
+
+  $ hg init amend-p1
+  $ cd amend-p1
+  $ touch a
+  $ hg ci -qAm0 a
+  $ echo foo > a
+  $ hg ci -m1
+  $ hg up -q 0
+  $ echo bar > a
+  $ hg ci -qm2
+  $ echo foo > a
+  $ hg ci -qm3 --amend
+
+  $ hg merge
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci --debug -m merge
+  reusing manifest from p1 (no file change)
+  committing changelog
+  updating the branch cache
+  committed changeset 3:314e5bc5adf5c58ea571efabe33eedba20a201aa
+  $ hg log -GT'{l} branch={branch}'
+  @    3:314e5bc5adf5 p=2,1 m=1:d33ea248bd73 f=[] branch=default
+  |\
+  | o  2:de9c64f226a3 p=0,-1 m=1:d33ea248bd73 f=["a"] branch=default
+  | |
+  o |  1:6a74aec01b3c p=0,-1 m=1:d33ea248bd73 f=["a"] branch=default
+  |/
+  o  0:487a0a245cea p=-1,-1 m=0:8515d4bfda76 f=["a"] branch=default
+  
+
+  $ cd ..
+  $ check_convert_identity amend-p1