tests: show the files fields of changelogs for many merges

I don't think there's coverage for many of the subtle cases, and I
found it hard to understand what the code is doing by reading it.  The
test takes 40s to run on a laptop, or 9s with --chg.

I have yet to find a description of what the files field is supposed
to be for merges. I thought it could be one of:
1. the files added/modified/removed relative to p1 (wouldn't seem
  useful, but `hg diff -c -r mergerev` has this behavior)
2. the files with filelog nodes not in either parent (i.e., what is
needed to create a bundle out of a commit)
3. the files added/removed/modified files by merge itself [1]

It's clearly not 1, because file contents merges are symmetric. It's
clearly not 2 because removed files and exec bit changes are
listed. It's also not 3 but I think it's intended to be 3 and the
differences are bugs.

Assuming 3, the test shows that, for merges, the list of files both
overapproximates and underapproximates. All the cases involve file
changes not in the filelog but in the manifest (existence of file
at revision, exec bit and file vs symlink).

I didn't look at all underapproximations, but they looked minor. The
two overapproximations are problematic though because they both cause
potentially long lists of files when merging cleanly.


[1] even what it means for the merge commit itself to change a file is
not completely trivial. A file in the merge being the same as in one
of the parent is too lax as it would consider that merges change
nothing when they revert all the changes done on one side. The
criteria used in the test and in the next commit for "merge didn't
touch a file" is:
- the parents and the merge all have the same file
- or, one parent didn't touch the file and the other parent contains
  the same file as the merge

Differential Revision: https://phab.mercurial-scm.org/D6612

tests/test-merge-combination.t

@@ -0,0 +1,220 b''
+This file shows what hg says are "modified" files for a merge commit
+(hg log -T {files}), somewhat exhaustively.
+It shows merges that involves files contents changing, and merges that
+involve executable bit changing, but not merges with multiple or zero
+merge ancestors, nor copies/renames, and nor identical file contents
+with different filelog revisions.
+
+genmerges is the workhorse. Given:
+- a range function describing the possible values for file a
+- a isgood function to filter out uninteresting combination
+- a createfile function to actually write the values for file a on the
+filesystem
+it print a series of lines that look like: abcd C: output of -T {files}
+describing the file a at respectively the base, p2, p1, merge
+revision. "C" indicates that hg merge had conflicts.
+  $ genmerges () {
+  >   for base in `range` -; do
+  >     for r1 in `range $base` -; do
+  >       for r2 in `range $base $r1` -; do
+  >         for m in `range $base $r1 $r2` -; do
+  >           line="$base$r1$r2$m"
+  >           isgood $line || continue
+  >           hg init repo
+  >           cd repo
+  >           make_commit () {
+  >             v=$1; msg=$2; file=$3;
+  >             if [ $v != - ]; then
+  >               createfile $v
+  >             else
+  >               if [ -f a ]
+  >               then rm a
+  >               else touch $file
+  >               fi
+  >             fi
+  >             hg commit -q -Am $msg || exit 123
+  >           }
+  >           echo foo > foo
+  >           make_commit $base base b
+  >           make_commit $r1 r1 c
+  >           hg up -r 0 -q
+  >           make_commit $r2 r2 d
+  >           hg merge -q -r 1 > ../output 2>&1
+  >           if [ $? -ne 0 ]; then rm -f *.orig; hg resolve -m --all -q; fi
+  >           if [ -s ../output ]; then conflicts=" C"; else conflicts="  "; fi
+  >           make_commit $m m e
+  >           if [ $m = $r1 ] && [ $m = $r2 ]
+  >           then expected=
+  >           elif [ $m = $r1 ]
+  >           then if [ $base = $r2 ]
+  >                then expected=
+  >                else expected=a
+  >                fi
+  >           elif [ $m = $r2 ]
+  >           then if [ $base = $r1 ]
+  >                then expected=
+  >                else expected=a
+  >                fi
+  >           else expected=a
+  >           fi
+  >           got=`hg log -r 3 --template '{files}\n' | tr --delete 'e '`
+  >           if [ "$got" = "$expected" ]
+  >           then echo "$line$conflicts: agree on \"$got\""
+  >           else echo "$line$conflicts: hg said \"$got\", expected \"$expected\""
+  >           fi
+  >           cd ../
+  >           rm -rf repo
+  >         done
+  >       done
+  >     done
+  >   done
+  > }
+
+All the merges of various file contents.
+
+  $ range () {
+  >   max=0
+  >   for i in $@; do
+  >     if [ $i = - ]; then continue; fi
+  >     if [ $i -gt $max ]; then max=$i; fi
+  >   done
+  >   $TESTDIR/seq.py `expr $max + 1`
+  > }
+  $ isgood () { true; }
+  $ createfile () {
+  >   if [ -f a ] && [ "`cat a`" = $1 ]
+  >   then touch $file
+  >   else echo $v > a
+  >   fi
+  > }
+
+  $ genmerges
+  1111  : agree on ""
+  1112  : agree on "a"
+  111-  : agree on "a"
+  1121  : agree on "a"
+  1122  : agree on ""
+  1123  : agree on "a"
+  112-  : agree on "a"
+  11-1  : hg said "", expected "a"
+  11-2  : agree on "a"
+  11--  : agree on ""
+  1211  : agree on "a"
+  1212  : agree on ""
+  1213  : agree on "a"
+  121-  : agree on "a"
+  1221  : agree on "a"
+  1222  : agree on ""
+  1223  : agree on "a"
+  122-  : agree on "a"
+  1231 C: agree on "a"
+  1232 C: agree on "a"
+  1233 C: agree on "a"
+  1234 C: agree on "a"
+  123- C: agree on "a"
+  12-1 C: agree on "a"
+  12-2 C: hg said "", expected "a"
+  12-3 C: agree on "a"
+  12-- C: agree on "a"
+  1-11  : hg said "", expected "a"
+  1-12  : agree on "a"
+  1-1-  : hg said "a", expected ""
+  1-21 C: agree on "a"
+  1-22 C: hg said "", expected "a"
+  1-23 C: agree on "a"
+  1-2- C: agree on "a"
+  1--1  : agree on "a"
+  1--2  : agree on "a"
+  1---  : agree on ""
+  -111  : agree on ""
+  -112  : agree on "a"
+  -11-  : agree on "a"
+  -121 C: agree on "a"
+  -122 C: agree on "a"
+  -123 C: agree on "a"
+  -12- C: agree on "a"
+  -1-1  : agree on ""
+  -1-2  : agree on "a"
+  -1--  : agree on "a"
+  --11  : agree on ""
+  --12  : agree on "a"
+  --1-  : agree on "a"
+  ---1  : agree on "a"
+  ----  : agree on ""
+
+All the merges of executable bit.
+
+  $ range () {
+  >   max=a
+  >   for i in $@; do
+  >     if [ $i = - ]; then continue; fi
+  >     if [ $i > $max ]; then max=$i; fi
+  >   done
+  >   if [ $max = a ]; then echo f; else echo f x; fi
+  > }
+  $ isgood () { case $line in *f*x*) true;; *) false;; esac; }
+  $ createfile () {
+  >   if [ -f a ] && (([ -x a ] && [ $v = x ]) || (! [ -x a ] && [ $v != x ]))
+  >   then touch $file
+  >   else touch a; if [ $v = x ]; then chmod +x a; else chmod -x a; fi
+  >   fi
+  > }
+
+#if execbit
+  $ genmerges
+  fffx  : agree on "a"
+  ffxf  : agree on "a"
+  ffxx  : agree on ""
+  ffx-  : agree on "a"
+  ff-x  : hg said "", expected "a"
+  fxff  : hg said "", expected "a"
+  fxfx  : hg said "a", expected ""
+  fxf-  : agree on "a"
+  fxxf  : agree on "a"
+  fxxx  : agree on ""
+  fxx-  : agree on "a"
+  fx-f  : hg said "", expected "a"
+  fx-x  : hg said "", expected "a"
+  fx--  : hg said "", expected "a"
+  f-fx  : agree on "a"
+  f-xf  : agree on "a"
+  f-xx  : hg said "", expected "a"
+  f-x-  : agree on "a"
+  f--x  : agree on "a"
+  -ffx  : agree on "a"
+  -fxf C: agree on "a"
+  -fxx C: hg said "", expected "a"
+  -fx- C: agree on "a"
+  -f-x  : hg said "", expected "a"
+  --fx  : agree on "a"
+#endif
+
+Files modified or cleanly merged, with no greatest common ancestors:
+
+  $ hg init repo; cd repo
+  $ touch a0 b0; hg commit -qAm 0
+  $ hg up -qr null; touch a1 b1; hg commit -qAm 1
+  $ hg merge -qr 0; rm b*; hg commit -qAm 2
+  $ hg log -r . -T '{files}\n'
+  b0 b1
+  $ cd ../
+  $ rm -rf repo
+
+A few cases of criss-cross merges involving deletions (listing all
+such merges is probably too much). Both gcas contain $files, so we
+expect the final merge to behave like a merge with a single gca
+containing $files.
+
+  $ hg init repo; cd repo
+  $ files="c1 u1 c2 u2"
+  $ touch $files; hg commit -qAm '0 root'
+  $ for f in $files; do echo f > $f; done; hg commit -qAm '1 gca1'
+  $ hg up -qr0; hg revert -qr 1 --all; hg commit -qAm '2 gca2'
+  $ hg up -qr 1; hg merge -qr 2; rm *1; hg commit -qAm '3 p1'
+  $ hg up -qr 2; hg merge -qr 1; rm *2; hg commit -qAm '4 p2'
+  $ hg merge -qr 3; echo f > u1; echo f > u2; rm -f c1 c2
+  $ hg commit -qAm '5 merge with two gcas'
+  $ hg log -r . -T '{files}\n' # expecting u1 u2
+  c1
+  $ cd ../
+  $ rm -rf repo