fast-export: allow user to request tags be marked with --mark-tags
Add a new option, --mark-tags, which will output mark identifiers with
each tag object. This improves the incremental export story with
--export-marks since it will allow us to record that annotated tags have
been exported, and it is also needed as a step towards supporting nested
tags.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>