Small fixes and features for fast-export and fast-import, mostly on
the fast-export side.

* en/fast-export-import:
  fast-export: add a --show-original-ids option to show original names
  fast-import: remove unmaintained duplicate documentation
  fast-export: add --reference-excluded-parents option
  fast-export: ensure we export requested refs
  fast-export: when using paths, avoid corrupt stream with non-existent mark
  fast-export: move commit rewriting logic into a function for reuse
  fast-export: avoid dying when filtering by paths and old tags exist
  fast-export: use value from correct enum
  git-fast-export.txt: clarify misleading documentation about rev-list args
  git-fast-import.txt: fix documentation for --quiet option
  fast-export: convert sha1 to oid