phases: update the phase set as we go during retract boundary
Apparently iterating over the `changed_revs` dictionary is very expensive.
On mozilla-try-2019-02-18, a perf::unbundle call with a 10 000 changesets
bundle gives give use the following timing.
e57d4b868a3e: 4.6 seconds
ac1c75188440: 102.5 seconds
prev-changeset: 30.0 seconds
this-changeset: 4.6 seconds
So, the performance regression is gone.
Once again: thanks to marvelous Python!