support pushing to a derived branch, but in the main repository

[jelmer]

See https://salsa.debian.org/jelmer/debian-janitor/-/issues/156 for background

It would be useful to support new publishing modes that function like push_derived and propose, but create a branch in the original repository rather than in the bots repository. Maybe this should be a flag in the publishing policy rather than altogether different modes - "derived_branches_in_origin: true" or something like that.

For example, it could create a "merge-upstream/3.3" branch in the main packaging repository that the maintainer can then easily find and pull into the main packaging branch at their own leisure.

Most of this is straightforward on the janitor/silver-platter side. The main challenge is around garbage collection - we'd presumably want to clean up these branches when they become obsolete.

See jelmer/silver-platter#15 for the matching silver-platter bug report - which is where the core logic should be.