grep: avoid accepting ambiguous revision

grep: avoid accepting ambiguous revision

Unlike other commands that take both revs and pathspecs without "--"
disamiguators only when the boundary is clear, "git grep" treated
what can be interpreted as a rev as-is, without making sure that it
could also have meant a pathspec.  E.g.

    $ git grep -e foo master

when 'master' is in the working tree, should have triggered an
ambiguity error, but it didn't, and searched in the tree of the
commit named by 'master'.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>