Correctly request force-pushing in a triangular workflow

[stefanhaller]

• PR Description

Some people push to a different branch (or even remote) than they pull from. One example is described in #3437. Our logic of when to request a force push is not appropriate for these workflows: we check the configured upstream branch for divergence, but that's the one you pull from. We should instead check the push-to branch for divergence.

This branch implements this, but in a very hacky way. The approach is probably fine, but we need to find better names for the added fields, and clean up the API of the models.Branch class. For now this is only meant as a quick proof of concept.

Fixes #3437.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.01% (target: -2.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (aa81e19)	50656	42634	84.16%
Head commit (45aa829)	50660 (+4)	42631 (-3)	84.15% (-0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3528)	8	8	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

See your quality gate settings    Change summary preferences

_{Codacy will stop sending the deprecated coverage status from June 5th, 2024. Learn more}