revision: add rev_input_given flag

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

revision.h

@@ -71,6 +71,13 @@ struct rev_info {
 	const char *def;
 	struct pathspec prune_data;
 
+	/*
+	 * Whether the arguments parsed by setup_revisions() included any
+	 * "input" revisions that might still have yielded an empty pending
+	 * list (e.g., patterns like "--all" or "--glob").
+	 */
+	int rev_input_given;
+
 	/* topo-sort */
 	enum rev_sort_order sort_order;