tests/test-double-merge.out

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

resolving manifests
 overwrite None partial False
 ancestor 310fd17130da local 2092631ce82b+ remote 7731dad1c2b9
 foo: versions differ -> m
 foo: remote copied to bar -> m
copying foo to bar
merging foo and bar
my foo@2092631ce82b+ other bar@7731dad1c2b9 ancestor foo@310fd17130da
merging foo
my foo@2092631ce82b+ other foo@7731dad1c2b9 ancestor foo@310fd17130da
0 files updated, 2 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
-- foo --
line 0
line 1
line 2-1
-- bar --
line 0
line 1
line 2-2