Updates to fast-import/export.

* en/fast-imexport-nested-tags:
  fast-export: handle nested tags
  t9350: add tests for tags of things other than a commit
  fast-export: allow user to request tags be marked with --mark-tags
  fast-export: add support for --import-marks-if-exists
  fast-import: add support for new 'alias' command
  fast-import: allow tags to be identified by mark labels
  fast-import: fix handling of deleted tags
  fast-export: fix exporting a tag and nothing else