Fix the history list still claiming to update after it was cleaned up - #686

Fixes #669: PBGitWindowWiringTests timing out waiting for the revision list to settle.

This was originally filed as half of #643, which has since been closed - its other half turned out to be #703, a different failure in the same test class, fixed by #702.

PBGitWindowWiringTests has timed out four times in the last 87 CI runs
(17 Sep master, 18 Sep #668, 19 Sep #683 and 19 Sep master), always on
arm64, always on ((revisionList.isUpdating) is false), across three
different test methods. The count is a floor: a run that someone re-runs
to green no longer reports the failure at all.

-cleanup cancels the walk and the graph queue, but cancelled operations
never run the completion blocks that call -finishedGraphing, so nothing
clears isUpdating and the flag stays set for good. tearDown cleans up
and then waits on that flag, so that wait cannot succeed at any timeout:
in the master failure the two assertions are exactly 30 seconds apart,
one per wait. Locally each of these tests settles in under a second, so
30 seconds of genuine walking was never the explanation.

• Report the list as settled once cleanup has stopped the walk and the
  graph queue, since that is exactly what the flag means
• Say whether the walk was still parsing, and how many graph operations
  were left, when a wait gives up, so the next failure names the cause

This does not claim to explain the other half. -finishedGraphing clears
the flag only when the walk has stopped parsing and the queue has
drained, and PBGitHistoryGrapher.graphCommits: returns early without
calling it at all when its commit list is empty or its thread was
cancelled - both are ways for the light to be left on. The new message is
there to say which one, rather than guessing now.

Test plan:

• The new test fails on master ("a list that was cleaned up still claims
  to be updating") and passes with the fix
• PBGitHistoryListCleanupTests and PBGitWindowWiringTests run green
  together, 7 tests, on arm64