tests/test-double-merge

merge: do early copy to deal with issue636

Without copies/renames, merges source names are 1:1 with their
targets. Copies and renames introduce the possibility that there will
be two merges with the same input but different output. By doing the
copy to the destination name before the merge, the actual merge
becomes 1:1 again, and no source is the input to two different merges.

- add a preliminary scan to applyupdates to do copies
- for the merge action, pass the old name (for finding ancestors) and
  the new name (for input to the merge) to filemerge
- eliminate the old post-merge copy
- lookup file contents from new name in filemerge
- pass new name to external merge helper
- report merge failure at new name
- add a test

#!/bin/sh

hg init repo
cd repo

echo line 1 > foo
hg ci -qAm 'add foo' -d "1000000 0"

# copy foo to bar and change both files
hg cp foo bar
echo line 2-1 >> foo
echo line 2-2 >> bar
hg ci -m 'cp foo bar; change both' -d "1000000 0"

# in another branch, change foo in a way that doesn't conflict with
# the other changes
hg up -qC 0
echo line 0 >| foo
hg cat foo >> foo
hg ci -m 'change foo' -d "1000000 0"

# we get conflicts that shouldn't be there
hg merge --debug

echo "-- foo --"
cat foo

echo "-- bar --"
cat bar