Merge branch 'sg/describe-contains'

"git describe" without argument defaulted to describe the HEAD
commit, but "git describe --contains" didn't.  Arguably, in a
repository used for active development, such defaulting would not
be very useful as the tip of branch is typically not tagged, but it
is better to be consistent.

* sg/describe-contains:
  describe --contains: default to HEAD when no commit-ish is given

builtin/describe.c

@@ -443,10 +443,10 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			if (pattern)
 				argv_array_pushf(&args, "--refs=refs/tags/%s", pattern);
 		}
-		while (*argv) {
-			argv_array_push(&args, *argv);
-			argv++;
-		}
+		if (argc)
+			argv_array_pushv(&args, argv);
+		else
+			argv_array_push(&args, "HEAD");
 		return cmd_name_rev(args.argc, args.argv, prefix);
 	}