Merge branch 'en/fast-export'

Merge branch 'en/fast-export'

* en/fast-export:
  fast-export: Document the fact that git-rev-list arguments are accepted
  Add new fast-export testcases
  fast-export: Add a --tag-of-filtered-object option for newly dangling tags
  fast-export: Do parent rewriting to avoid dropping relevant commits
  fast-export: Make sure we show actual ref names instead of "(null)"
  fast-export: Omit tags that tag trees
  fast-export: Set revs.topo_order before calling setup_revisions