merge: allow merging going backwards

New behavior is generally superior and more correct, except possibly
with regards to missing files. hg up . is now effectively a no-op,
which is probably the desired behavior for people expecting to move to
tip, but may surprise people who were expecting deleted files to
reappear.

case 1: update to .

a-w -> a-w

classic: ancestor a
 missing     recreated     right?
 rmed        recreated     WRONG
 added       forgotten     WRONG
 changed     preserved     RIGHT
 conflicted  can't happen

backward merge: ancestor a  (NO EFFECT)
 missing     missing      wrong?
 rm'ed       rm'ed        RIGHT
 added       preserved	  RIGHT
 changed     preserved    RIGHT
 conflicted  can't happen

case 2: update to ancestor of .

a-b-w -> b-w
          \
           a

classic:  ancestor a
 missing       recreated        right?
 rmed          recreated        wrong?
 added         forgotten        wrong?
 changed       preserved        RIGHT
 conflicted    preserved        wrong?

backwards merge: ancestor b
 missing       missing or conflict  right?
 rm'ed         missing or conflict  right?
 changed       preserved            RIGHT
 conflicted    merge                RIGHT
 added         preserved            right?

mercurial/merge.py

@@ -131,11 +131,14 b' def manifestmerge(repo, p1, p2, pa, over'
 
     m1 = p1.manifest()
     m2 = p2.manifest()
-    ma = pa.manifest()
     backwards = (pa == p2)
 
-    if backwards or overwrite:
+    if overwrite:
         ma = m1
+    elif backwards:
+        ma = p1.p1().manifest()
+    else:
+        ma = pa.manifest()
 
     action = []
     copy, copied, diverge = {}, {}, {}

tests/test-custom-filters

@@ -43,7 +43,7 b' hg cat stuff.txt'
 
 echo '% Fresh checkout:'
 rm stuff.txt
-hg up
+hg up -C
 cat stuff.txt
 echo >> stuff.txt <<EOF
 Very very carefully.

tests/test-encode

@@ -28,7 +28,7 b' echo %% uncompress our working dir copy'
 gunzip < a.gz
 
 rm a.gz
-hg co
+hg co -C
 
 echo %% uncompress our new working dir copy
 gunzip < a.gz

tests/test-execute-bit.out

@@ -3,6 +3,6 b' 79abf14474dc tip'
 % make sure we notice the change of mode if the cached size == -1
 n   0         -1 unset               a
 M a
-1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+0 files updated, 0 files merged, 0 files removed, 0 files unresolved
 d69afc33ff8a
 not executable -- whew

tests/test-keyword

@@ -117,7 +117,7 b' hg status'
 
 rm sym a b
 echo % update
-hg update
+hg update -C
 echo % cat
 cat a b
 

tests/test-merge-local.out

@@ -22,7 +22,9 b' merging zzz2_merge_bad'
 merging zzz2_merge_bad failed!
 3 files updated, 1 files merged, 2 files removed, 1 files unresolved
 use 'hg resolve' to retry unresolved file merges
-2 files updated, 0 files merged, 3 files removed, 0 files unresolved
+merging zzz1_merge_ok
+merging zzz2_merge_bad
+2 files updated, 2 files merged, 3 files removed, 0 files unresolved
 --- a/zzz1_merge_ok
 +++ b/zzz1_merge_ok
 +new first line
@@ -32,7 +34,6 b' 2 files updated, 0 files merged, 3 files'
 +another last line
 M zzz1_merge_ok
 M zzz2_merge_bad
-? zzz2_merge_bad.orig
 # local merge with conflicts
 merging zzz1_merge_ok
 merging zzz2_merge_bad
@@ -40,7 +41,9 b' warning: conflicts during merge.'
 merging zzz2_merge_bad failed!
 3 files updated, 1 files merged, 2 files removed, 1 files unresolved
 use 'hg resolve' to retry unresolved file merges
-2 files updated, 0 files merged, 3 files removed, 0 files unresolved
+merging zzz1_merge_ok
+merging zzz2_merge_bad
+2 files updated, 2 files merged, 3 files removed, 0 files unresolved
 --- a/zzz1_merge_ok
 +++ b/zzz1_merge_ok
 +new first line
@@ -52,7 +55,6 b' 2 files updated, 0 files merged, 3 files'
 +new last line
 M zzz1_merge_ok
 M zzz2_merge_bad
-? zzz2_merge_bad.orig
 # local merge without conflicts
 merging zzz1_merge_ok
 4 files updated, 1 files merged, 2 files removed, 0 files unresolved

tests/test-up-local-change.out

@@ -38,9 +38,15 b' summary: 2'
 resolving manifests
  overwrite False partial False
  ancestor 33aaa84a386b local 802f095af299+ remote 33aaa84a386b
+ a: versions differ -> m
  b: other deleted -> r
+preserving a for resolve of a
 removing b
-0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+picked tool 'true' for a (binary False symlink False)
+merging a
+my a@802f095af299+ other a@33aaa84a386b ancestor a@33aaa84a386b
+ premerge successful
+0 files updated, 1 files merged, 1 files removed, 0 files unresolved
 changeset:   0:33aaa84a386b
 user:        test
 date:        Mon Jan 12 13:46:40 1970 +0000

tests/test-win32text

@@ -117,7 +117,7 b' cat .hg/hgrc'
 echo
 
 rm f3 f4.bat bin
-hg co 2>&1 | python -c 'import sys, os; sys.stdout.write(sys.stdin.read().replace(os.getcwd(), "...."))'
+hg co -C 2>&1 | python -c 'import sys, os; sys.stdout.write(sys.stdin.read().replace(os.getcwd(), "...."))'
 python print.py < bin
 python print.py < f3
 python print.py < f4.bat
@@ -152,7 +152,7 b' cat .hg/hgrc'
 echo
 
 rm f3 f4.bat bin
-hg co 2>&1 | python -c 'import sys, os; sys.stdout.write(sys.stdin.read().replace(os.getcwd(), "...."))'
+hg co -C 2>&1 | python -c 'import sys, os; sys.stdout.write(sys.stdin.read().replace(os.getcwd(), "...."))'
 python print.py < bin
 python print.py < f3
 python print.py < f4.bat

tests/test-win32text.out

@@ -314,7 +314,7 b' win32text ='
 [encode]
 ** = !
 
-3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+4 files updated, 0 files merged, 0 files removed, 0 files unresolved
 hello<NUL><CR><LF>
 some<LF>text<LF>
 rem empty<LF>