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>

remote.h

@@ -51,6 +51,8 @@ struct refspec {
 	char *dst;
 };
 
+extern const struct refspec *tag_refspec;
+
 struct ref *alloc_ref(unsigned namelen);
 
 struct ref *copy_ref_list(const struct ref *ref);