A few fast export fixups -- round 2

This patch series fixes a few fast-export bugs I have come across, plus some new testcases to verify the fixes, 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

Elijah Newren (6):
  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             |   72 +++++++++++++++++++++++++++++++++---
 revision.c                        |   10 +----
 revision.h                        |    8 ++++
 t/t9301-fast-export.sh            |   63 +++++++++++++++++++++++++++++++-
 5 files changed, 155 insertions(+), 15 deletions(-)