tests/test-backwards-remove.t

githelp: clean up reflog help

This referenced commands that don't exist in core. The new help
isn't great since it references an experimental extension. But
it is better than nothing.

While we're here, also add test coverage.

Differential Revision: https://phab.mercurial-scm.org/D1727

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a