xdiff/xmerge.c: use memset() instead of explicit for-loop
xdiff/xmerge.c: use memset() instead of explicit for-loop

memset() is heavily optimized, and resulting assembler code
is about 150 lines less for that file.

Signed-off-by: Alexey Mahotkin <f2288fd6431412072d154da4fb5a288cb472d310@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git