builtin-prune: memory diet.

Somehow we forgot to turn save_commit_buffer off while walking
the reachable objects.  Releasing the memory for commit object
data that we do not use matters for large projects (for example,
about 90MB is saved while traversing linux-2.6 history).

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-prune.c

@@ -253,6 +253,8 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 		usage(prune_usage);
 	}
 
+	save_commit_buffer = 0;
+
 	/*
 	 * Set up revision parsing, and mark us as being interested
 	 * in all object types, not just commits.