Make git-resolve-script only write MERGE_HEAD if a merge actually
occurred. All merge failures leave ORIG_HEAD and LAST_MERGE
behind (instead of ORIG_HEAD and MERGE_HEAD).

Use git-rev-parse to expand arguments (and check for bad ones).
Signed-off-by: NDan Holmsand <holmsand@gmail.com>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>