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.
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee (1 drafts)
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee (1 drafts)
4 files updated, 0 files merged, 0 files removed, 0 files unresolved