grep: treat revs the same for --untracked as for --no-index

git-grep has always disallowed grepping in a tree (as
opposed to the working directory) with both --untracked
and --no-index. But we traditionally did so by first
collecting the revs, and then complaining when any were
provided.

The --no-index option recently learned to detect revs
much earlier. This has two user-visible effects:

  - we don't bother to resolve revision names at all. So
    when there's a rev/path ambiguity, we always choose to
    treat it as a path.

  - likewise, when you do specify a revision without "--",
    the error you get is "no such path" and not "--untracked
    cannot be used with revs".

The rationale for doing this with --no-index is that it is
meant to be used outside a repository, and so parsing revs
at all does not make sense.

This patch gives --untracked the same treatment. While it
_is_ meant to be used in a repository, it is explicitly
about grepping the non-repository contents. Telling the user
"we found a rev, but you are not allowed to use revs" is
not really helpful compared to "we treated your argument as
a path, and could not find it".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7810-grep.sh

@@ -1032,7 +1032,7 @@ test_expect_success 'grep --no-index pattern -- path' '
 
 test_expect_success 'grep --no-index complains of revs' '
 	test_must_fail git grep --no-index o master -- 2>err &&
-	test_i18ngrep "no-index cannot be used with revs" err
+	test_i18ngrep "cannot be used with revs" err
 '
 
 test_expect_success 'grep --no-index prefers paths to revs' '