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

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>