A few fast export fixups -- round 3

This patch series fixes a few fast-export bugs I have come across, plus some new testcases to verify the fixes, a testcase to document a pre-existing failure, and finally a small addition to the documentation.

Changes since v1:
  - fixed test issues identified by Johannes Sixt and Stephen Boyd
  - squashed patch #2 into patch #1

Changes since v2:
  - several changes and improvements suggested by Junio (moving code
    to different functions, avoiding exporting functionality from
    revision.c, improved commit messages)
  - new patch to ensure invariants created by setup_revisions are
    respected (set revs flags before calling setup_revisions, rather
    than after)
  - now also catches and omits tags of tags of trees, tags of tags of
    tags of trees, etc.
  - remove some now-obsoleted code
  - There are two new cases I'm aware of in which fast-export produces
    broken "(null)" refs; I'm not sure how/where to fix these (or even
    what correct behavior is in one of the cases), but they are no worse
    than before and I've at least added a test documenting that we know
    that there's a case that is (still) broken.

Elijah Newren (7):
      fast-export: Set revs.topo_order before calling setup_revisions
      fast-export: Omit tags that tag trees
      fast-export: Make sure we show actual ref names instead of "(null)"
      fast-export: Do parent rewriting to avoid dropping relevant commits
      fast-export: Add a --tag-of-filtered-object option for newly dangling tags
      Add new fast-export testcases
      fast-export: Document the fact that git-rev-list arguments are accepted

 Documentation/git-fast-export.txt |   17 +++++++
 builtin-fast-export.c             |   82 +++++++++++++++++++++++++++++---
 t/t9301-fast-export.sh            |   96 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 187 insertions(+), 8 deletions(-)