tests/test-backwards-remove.t

revlog: add exception when linkrev == nullrev

When we deployed the latest crew mercurial to our users, a few of them
had issues where a filelog would have an entry with a -1 linkrev. This
caused operations like rebase and amend to create a bundle containing the
entire repository, which took a long time.

I don't know what the issue is, but adding this check should prevent repos
from getting in this state, and should help us pinpoint the issue next time
it happens.

  $ 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