builtin-fast-export: Add importing and exporting of revision marks

builtin-fast-export: Add importing and exporting of revision marks

This adds the --import-marks and --export-marks to fast-export. These import
and export the marks used to for all revisions exported in a similar fashion
to what fast-import does. The format is the same as fast-import, so you can
create a bidirectional importer / exporter by using the same marks file on
both sides.

Signed-off-by: Pieter de Bie <pdebie@ai.rug.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>