Normally a caller that invokes setup_revisions() has to
check rev.pending to see if anything was actually queued for
the traversal. But they can't tell the difference between
two cases:

  1. The user gave us no tip from which to start a
     traversal.

  2. The user tried to give us tips via --glob, --all, etc,
     but their patterns ended up being empty.

Let's set a flag in the rev_info struct that callers can use
to tell the difference.  We can set this from the
init_all_refs_cb() function.  That's a little funny because
it's not exactly about initializing the "cb" struct itself.
But that function is the common setup place for doing
pattern traversals that is used by --glob, --all, etc.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>