test-mergesort: add sort subcommand

Give the code for sorting a text file its own sub-command.  This allows
extending the helper, which we'll do in the following patches.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-mergesort.c

@@ -23,7 +23,7 @@ static int compare_strings(const void *a, const void *b)
 	return strcmp(x->text, y->text);
 }
 
-int cmd__mergesort(int argc, const char **argv)
+static int sort_stdin(void)
 {
 	struct line *line, *p = NULL, *lines = NULL;
 	struct strbuf sb = STRBUF_INIT;
@@ -49,3 +49,10 @@ int cmd__mergesort(int argc, const char **argv)
 	}
 	return 0;
 }
+
+int cmd__mergesort(int argc, const char **argv)
+{
+	if (argc == 2 && !strcmp(argv[1], "sort"))
+		return sort_stdin();
+	usage("test-tool mergesort sort");
+}