Have a constant extern refspec for "--tags"

The refspec refs/tags/*:refs/tags/* is sufficiently common and generic
to merit having a constant instead of generating it as needed.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.c b/remote.c
index 2d9af40..9cb40af 100644
--- a/remote.c
+++ b/remote.c
@@ -2,6 +2,15 @@
 #include "remote.h"
 #include "refs.h"
 
+static struct refspec s_tag_refspec = {
+	0,
+	1,
+	"refs/tags/",
+	"refs/tags/"
+};
+
+const struct refspec *tag_refspec = &s_tag_refspec;
+
 struct counted_string {
 	size_t len;
 	const char *s;