tests/test-absorb-unfinished.t

remotefilelog: reduce probability of race-condition in remotefilelog tests

ca1014ad3de4 introduced a new parameter `ensurestart` to speed up
remotefilelog background processes start. Unfortunately it seems to have
increased the possibility of race-conditions in remotefilelog tests testing
those background processes.

With `ensurestart=False`, it seems that it's more probable to enter in a race
condition with `debugwaitonprefetch` and `debugwaitonrepack` in remotefilelog
background tests. Our CI seems to have a high probability of triggering this
race condition so make it configurable to ensure tests are stable.

Differential Revision: https://phab.mercurial-scm.org/D6772

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > absorb=
  > EOF

Abort absorb if there is an unfinished operation.

  $ hg init abortunresolved
  $ cd abortunresolved

  $ echo "foo1" > foo.whole
  $ hg commit -Aqm "foo 1"

  $ hg update null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo "foo2" > foo.whole
  $ hg commit -Aqm "foo 2"

  $ hg --config extensions.rebase= rebase -r 1 -d 0
  rebasing 1:c3b6dc0e177a "foo 2" (tip)
  merging foo.whole
  warning: conflicts while merging foo.whole! (edit, then use 'hg resolve --mark')
  unresolved conflicts (see hg resolve, then hg rebase --continue)
  [1]

  $ hg --config extensions.rebase= absorb
  abort: rebase in progress
  (use 'hg rebase --continue' or 'hg rebase --abort')
  [255]