gc: --prune prunes unreferenced objects.

Brandon Casey correctly points out that we repack with -A without --prune
and with -a with --prune, so it is not just unreferenced loose objects
that are pruned away when the option is given.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-gc.c

@@ -175,7 +175,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	char buf[80];
 
 	struct option builtin_gc_options[] = {
-		OPT_BOOLEAN(0, "prune", &prune, "prune unreferenced loose objects"),
+		OPT_BOOLEAN(0, "prune", &prune, "prune unreferenced objects"),
 		OPT_BOOLEAN(0, "aggressive", &aggressive, "be more thorough (increased runtime)"),
 		OPT_BOOLEAN(0, "auto", &auto_gc, "enable auto-gc mode"),
 		OPT_END()