We special-case "git log :/foo" to work when "foo" exists in
the working tree. But :^ (and its alias :!) do not get the
same treatment, requiring the user to supply a
disambiguating "--". Let's make them work without requiring
the user to type the "--".
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>