Make git-rev-list and git-rev-parse argument parsing stricter

If you pass it a filename without the "--" marker to separate it from
revision information and flags, we now require that the file in question
actually exists. This makes mis-typed revision information not be silently
just considered a strange filename.

With the "--" marker, you can continue to pass in filenames that do not
actually exists - useful for querying what happened to a file that you
no longer have in the repository.

[ All scripts should use the "--" format regardless, to make things
  unambiguous. So this change should not affect any existing tools ]

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