tests/test-backwards-remove.t

revset: raise ValueError when calling min or max on empty smartset

min([]) raise a ValueError, we do the same thing in smartset.min() and
smartset.max() for the sake of consistency.

The min/amax test are greatly improved in the process to prevent this familly
of regression

  $ 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