fix overstrict :<path> diagnosis

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>