[PATCH] Make git-update-cache --refresh fail if update/merge needed.

[PATCH] Make git-update-cache --refresh fail if update/merge needed.

Scripts may find it useful if they do not have to parse the
output from the command but just can rely on its exit status.

Earlier both Linus and myself thought this would be necessary to
make git-prune-script safer but it turns out that the issue was
somewhere else and not related to what this patch addresses.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>