Updates transport-helper, fast-import and fast-export to allow the
ref mapping and ref deletion in a way similar to the natively
supported transports.

* fc/transport-helper-fixes:
  remote-bzr: support the new 'force' option
  test-hg.sh: tests are now expected to pass
  transport-helper.c: do not overwrite forced bit
  transport-helper: check for 'forced update' message
  transport-helper: add 'force' to 'export' helpers
  transport-helper: don't update refs in dry-run
  transport-helper: mismerge fix