filter-branch.sh: support nearly proper tag name filtering

filter-branch.sh: support nearly proper tag name filtering

Add support for creating a new tag object and retaining the tag message,
author, and date when rewriting tags. The gpg signature, if one exists,
will be stripped.

This adds nearly proper tag name filtering to filter-branch. Proper tag
name filtering would include the ability to change the tagger, tag date,
tag message, and _not_ strip a gpg signature if the tag did not change.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>