This makes diff-tree-helper handle ("warn about") unmerged path entries

    U <path> <record-terminator>

This is emitted once per unmerged path, no matter how many unmerged
stages there are. 
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>