SCMs have ways to say "I want diff only this particular file",
or "I want diff files under this directory".  This patch teaches
show-diff to do something similar.  Without command line
arguments, it still examines everything in the dircache as
before.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>