git-mv: shrink usage, no usage on error

Small fixes to be consistent with other git scripts:
- usage message is only about options and arguments
- on error, exit(1) without the usage message

Additionally, "beautifies" output with -n a little bit

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>