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

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>