tests/test-issue5979.t

branchcache: do not accept "empty update"

This currently does not happens and it will be simpler that is remains that way.
If all update do something, we will be able to simply declare, in a later
changesets, that all update to result in a dirty branchcache.

  $ hg init r1
  $ cd r1
  $ hg ci --config ui.allowemptycommit=true -m c0
  $ hg ci --config ui.allowemptycommit=true -m c1
  $ hg ci --config ui.allowemptycommit=true -m c2
  $ hg co -q 0
  $ hg ci --config ui.allowemptycommit=true -m c3
  created new head
  $ hg co -q 3
  $ hg merge --quiet
  $ hg ci --config ui.allowemptycommit=true -m c4

  $ hg log -G -T'{desc}'
  @    c4
  |\
  | o  c3
  | |
  o |  c2
  | |
  o |  c1
  |/
  o  c0
  

  >>> from mercurial import hg
  >>> from mercurial import ui as uimod
  >>> repo = hg.repository(uimod.ui())
  >>> for anc in repo.changelog.ancestors([4], inclusive=True):
  ...   print(anc)
  4
  3
  2
  1
  0