phases: avoid a potentially costly dictionary interation in some case
If we retract for the draft phase, there is not non-public item to be retracted
and we can skip this part. This part is was apparently super costly thanks to
Python.
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
this-changeset: 30.0 seconds
So we recovered about ⅔ of the regression, the next changeset will give us the
rest back.
skip ignored directories if -i or --all not specified
$ hg init t
$ cd t
$ echo 'ignored' > .hgignore
$ hg ci -qA -m init -d'2 0'
$ mkdir ignored
$ ls
ignored
$ hg purge -v --no-confirm
$ ls
ignored