Avoid ambiguity between refname and filename in rev-parse

Avoid ambiguity between refname and filename in rev-parse

Although it really is very convenient, not requiring explicit
'-r' option to name revs is sometimes ambiguous.

Usually we allow a "--" to say where a filename starts when it
_is_ ambiguous.  However, we fail that at times. In particular,
git-rev-parse fails it.

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