prune: show progress while marking reachable objects

prune already shows progress meter while pruning. The marking part may
take a few seconds or more, depending on repository size. Show
progress meter during this time too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/reachable.h b/reachable.h
index 4075181..5d082ad 100644
--- a/reachable.h
+++ b/reachable.h
@@ -1,6 +1,7 @@
 #ifndef REACHEABLE_H
 #define REACHEABLE_H
 
-extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog);
+struct progress;
+extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog, struct progress *);
 
 #endif