remote: add a macro for "refs/tags/*:refs/tags/*"

Add a macro with the refspec string "refs/tags/*:refs/tags/*". There's
been a pre-defined struct version of this since e0aaa29ff3 ("Have a
constant extern refspec for "--tags"", 2008-04-17), but nothing that
could be passed to e.g. add_fetch_refspec().

This will be used in subsequent commits to avoid hardcoding this
string in multiple places.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -22,6 +22,7 @@ static struct refspec s_tag_refspec = {
 	"refs/tags/*"
 };
 
+/* See TAG_REFSPEC for the string version */
 const struct refspec *tag_refspec = &s_tag_refspec;
 
 struct counted_string {

remote.h

@@ -297,4 +297,6 @@ extern int parseopt_push_cas_option(const struct option *, const char *arg, int
 extern int is_empty_cas(const struct push_cas_option *);
 void apply_push_cas(struct push_cas_option *, struct remote *, struct ref *);
 
+#define TAG_REFSPEC "refs/tags/*:refs/tags/*"
+
 #endif