request-pull: explicitly ask tags/$name to be pulled

When asking for a tag to be pulled, disambiguate by leaving tags/ prefix
in front of the name of the tag. E.g.

    ... in the git repository at:

      git://example.com/git/git.git/ tags/v1.2.3

    for you to fetch changes up to 123456...

This way, older versions of "git pull" can be used to respond to such a
request more easily, as "git pull $URL v1.2.3" did not DWIM to fetch
v1.2.3 tag in older versions. Also this makes it clearer for humans that
the pull request is made for a tag and he should anticipate a signed one.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/using-signed-tag-in-pull-request.txt

@@ -109,7 +109,7 @@ The resulting msg.txt file begins like so:
 
  are available in the git repository at:
 
-   example.com:/git/froboz.git frotz-for-xyzzy
+   example.com:/git/froboz.git tags/frotz-for-xyzzy
 
  for you to fetch changes up to 703f05ad5835c...:
 
@@ -141,7 +141,7 @@ After receiving such a pull request message, the integrator fetches and
 integrates the tag named in the request, with:
 
 ------------
- $ git pull example.com:/git/froboz.git/ frotz-for-xyzzy
+ $ git pull example.com:/git/froboz.git/ tags/frotz-for-xyzzy
 ------------
 
 This operation will always open an editor to allow the integrator to fine