workingctx: add a way for extensions to run code at status fixup time Some extensions like fsmonitor need to run code after dirstate.status is called, but while the wlock is held. The extensions could grab the wlock again, but that has its own peculiar race issues. For example, fsmonitor would not like its state to be written out if the dirstate has changed underneath (see issue5581 for what can go wrong in that sort of case). To protect against these sorts of issues, allow extensions to declare that they would like to run some code to run at fixup time. fsmonitor will switch to using this in the next patch in the series.