git-tag: introduce --cleanup option

git-tag: introduce --cleanup option

Normally git tag strips tag message lines starting with '#', trailing
spaces from every line and empty lines from the beginning and end.

--cleanup allows to select different cleanup modes for tag message.
It provides the same interface as --cleanup option in git-commit.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>