Teach revision walking machinery to walk multiple times sequencially

Previously it was not possible to iterate revisions twice using the
revision walking api. We add a reset_revision_walk() which clears the
used flags. This allows us to do multiple sequencial revision walks.

We add the appropriate calls to the existing submodule machinery doing
revision walks. This is done to avoid surprises if future code wants to
call these functions more than once during the processes lifetime.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -2061,6 +2061,11 @@ static void set_children(struct rev_info *revs)
 	}
 }
 
+void reset_revision_walk(void)
+{
+	clear_object_flags(SEEN | ADDED | SHOWN);
+}
+
 int prepare_revision_walk(struct rev_info *revs)
 {
 	int nr = revs->pending.nr;