grep: avoid resolving revision names in --no-index case

grep: avoid resolving revision names in --no-index case

We disallow the use of revisions with --no-index, but we
don't actually check and complain until well after we've
parsed the revisions.

This is the cause of a few problems:

 1. We shouldn't be calling get_sha1() at all when we aren't
    in a repository, as it might access the ref or object
    databases. For now, this should generally just return
    failure, but eventually it will become a BUG().

 2. When there's a "--" disambiguator and you're outside a
    repository, we'll complain early with "unable to resolve
    revision". But we can give a much more specific error.

 3. When there isn't a "--" disambiguator, we still do the
    normal rev/path checks. This is silly, as we know we
    cannot have any revs with --no-index. Everything we see
    must be a path.

    Outside of a repository this doesn't matter (since we
    know it won't resolve), but inside one, we may complain
    unnecessarily if a filename happens to also match a
    refname.

This patch skips the get_sha1() call entirely in the
no-index case, and behaves as if it failed (with the
exception of giving a better error message).

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