Always lock the index when we begin the merge, before we write
any of the metdata files. This prevents a race where another
client may run a commit after we have written the MERGE_HEAD but
before we have updated the index, which will produce a merge
commit that is treesame to one parent. The merge will finish and
update the index and the resultant commit would not be a merge at
all.
Correct the merge failed cleanup test. Merge data should not be
cleaned up on conflicts, only on actual failure. And ORIG_HEAD
should not be removed at all.
