fix overstrict :<path> diagnosis

Given "git log :", we get a disambiguation message that tries to be helpful and yet totally misses the point, i.e. $ git log : fatal: Path '' does not exist (neither on disk nor in the index). $ git log :/ fatal: Path '/' exists on disk, but not in the index. An empty path nor anything that begins with '/' cannot possibly in the index, and it is wrong to guess that the user might have meant to access such an index entry. It should yield the same error message as "git log '*.c'", i.e. $ git log '*.c' fatal: ambiguous argument '*.c': unknown revision or path not in the working tree. Use '--' to separate paths from revisions Signed-off-by: Junio C Hamano <gitster@pobox.com>