rev-list: check reflog_info before showing usage

rev-list: check reflog_info before showing usage

When git-rev-list sees no pending commits, it shows a usage
message. This works even when reflog-walking is requested,
because the reflog-walk code currently puts the reflog tips
into the pending queue.

In preparation for refactoring the reflog-walk code, let's
explicitly check whether we have any reflogs to walk. For
now this is a noop, but the existing reflog tests will make
sure that it kicks in after the refactoring. Likewise, we'll
add a test that "rev-list -g" without specifying any reflogs
continues to fail (so that we know our check does not kick
in too aggressively).

Note that the implementation needs to go into its own
sub-function, as the walk code does not expose its innards
outside of reflog-walk.c.

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