Add merge menu with conflict resolver

[lmnek]

Implements #2026. I also tried to address issues mentioned in the #3477 PR.

Previously, pressing M opened an external merge tool. Now it opens a merge options menu that allows selecting all conflicts in chosen files as ours (HEAD), theirs (incoming), or union (both), while still providing access to the external merge tool.

This uses git-merge-file for a 3-way merge with the --ours, --theirs, and --union flags. This approach avoids the issue mentioned in #1608 (reply in thread), and correctly applies the chosen conflict resolutions while preserving changes from other branches. The command is executed with --object-id, which requires object IDs obtained via rev-parse, instead of relying on the standard version that works with full saved files.

Disclaimer: On my machine, the tests pass inconsistently. Sometimes they succeed, sometimes fail with errors such as POTENTIAL DEADLOCK: Recursive locking or Inconsistent locking. I haven’t yet identified the root cause of this issue.

PR Description

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

O wow, this looks really nice at first glance. It might take me a while to review this, as I'm rather busy right now. Feel free to ping me if you haven't heard back after a week or two.

Disclaimer: On my machine, the tests pass inconsistently. Sometimes they succeed, sometimes fail with errors such as POTENTIAL DEADLOCK: Recursive locking or Inconsistent locking.

My guess is that you have go 1.25 installed locally. This is a known problem, I didn't have time to look into this yet. If you can use 1.24 to run the tests, it should be more reliable.