Draft: PoC: Add enhanced merge method support to merged results
What does this MR do and why?
Please treat this as a Proof of Concept for now:
- Think big picture
- Look for holes in the overall approach
- Avoid nitpicks and ignore test gaps
Prior to this change, the merge ref was always as a standard merge of the source branch and target branch, regardless of the merge method. With this change, we add a feature flag and project setting to allow the merged result will more closely follow the merge request and project preferences as follows:
- Squash if enabled on the MR
- Rebase if the project merge method is fast-forward or semi-linear
- Create a merge commit if standard merge or semi-linear
Behind the scenes, we actually implement this at the mergeability check
level, since they share the ref refs/merge-requests/:iid/merge, and
merged result pipelines create the ref via the mergeability check.
The project setting "Always use merge commit" is introduced for Merged Result Pipelines, enabled by default, to avoid a breaking change to users pipeline configuration. But as we are also working on supporting automatic rebase on merge, we will need to reconsider this setting:
- If automatic rebase is enabled on merge, then the mergeability check should always perform a rebase to detect rebase-specific conflicts.
- This means we either need to deprecate this setting or use separate refs for the mergeability check and merged result pipelines.
Part of #421025
Screenshots or screen recordings
Screenshots are required for UI changes, and strongly recommended for all other merge requests.
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How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
MR acceptance checklist
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