Merge branch 'master' of git://repo.or.cz/git/fastimport

* 'master' of git://repo.or.cz/git/fastimport:
  Add a Tips and Tricks section to fast-import's manual.
  Don't crash fast-import if the marks cannot be exported.
  Dump all refs and marks during a checkpoint in fast-import.
  Teach fast-import how to sit quietly in the corner.
  Teach fast-import how to clear the internal branch content.
  Minor timestamp related documentation corrections for fast-import.