revision parsing: make "rev -- paths" checks stronger.

revision parsing: make "rev -- paths" checks stronger.

If you don't have a "--" marker, then:

 - all of the arguments we are going to assume are pathspecs
   must exist in the working tree.

 - none of the arguments we parsed as revisions could be
   interpreted as a filename.

so that there really isn't any possibility of confusion in case
somebody does have a revision that looks like a pathname too.

The former rule has been in effect; this implements the latter.

Signed-off-by: Junio C Hamano <junkio@cox.net>