merge-tree: provide a list of which files have conflicts
Callers of `git merge-tree --write-tree` will often want to know which
files had conflicts. While they could potentially attempt to parse the
CONFLICT notices printed, those messages are not meant to be machine
readable. Provide a simpler mechanism of just printing the files (in
the same format as `git ls-files` with quoting, but restricted to
unmerged files) in the output before the free-form messages.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>