[PATCH] Add "-q" option to show-diff.c

This adds the '-q' option for show-diff.c to squelch complaints for
missing files.

It is handy if you want to run it in the merge temporary directory after
running merge-trees with its minimum checkout mode, which is the
default, because you would not find any files other than the ones that
needs human validation after the merge there.

It also fixes the argument parsing bug Paul Mackerras noticed in
<16991.42305.118284.139777@cargo.ozlabs.ibm.com> but slightly
differently.

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