Push any newly imported branches simultaneously with any new import tags
and atomically. This prevents any partial update and restores importer
idempotency.
This requires support for --atomic. It does exist on Xenial and seems to
work with Launchpad. I think this should be sufficient for our purposes.
If parent gets set for one series, it ends up being set for all of them,
which is wrong. Deconflate the parent requested by the caller from the
parent selected for use for a particular series by calling the latter
series_parent instead.
git ubuntu submit: add --reviewer option to request MP reviews
Default is ~canonical-server.
Note there is some issue here with LP and JSON decoding errors. For now,
they will be marked as "unknown error" in the catch-all, but still needs
investigation.