request-pull: use the annotated tag contents

request-pull: use the annotated tag contents

The integrator tool will start allowing to pull a signed or an annotated
tag, i.e.

    $ git pull $there tags/for-linus

and the description in the tag is used to convey a meaningful message from
the lieutenant to the integrator to justify the history being pulled.

Include the message in the pull request e-mail, as the same information is
useful in this context, too. It would encourage the lieutenants to write
meaningful messages in their signed tags.

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