This really just splits things up partially, and creates the
interface to set things up by parsing the command line.

No real code changes so far, although the parsing of filenames is a bit
stricter. In particular, if there is a "--", then we do not accept any
filenames before it, and if there isn't any "--", then we check that _all_
paths listed are valid, not just the first one.

The new argument parsing automatically also gives us "--default" and
"--not" handling as in git-rev-parse.

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