Overview

Closes #6960

Description

This PR adds a feature-flag to test whether the commits to be reapplied as part of the rebase will apply cleanly, and report if that isn't the case - this is an indication there will be conflicts to resolve. There's a bunch of work hardening, polishing and testing still to go but I thought I'd open this up early to share how this solution works.

First, what problems were there to address:

• we need a reliable way to test the commits without performing the whole rebase itself
• we want to get feedback reasonably quickly if there will problems
• we want to avoid making changes to the working directory, the index or the current branch

I've settled on a workflow here that uses Git worktrees, which we don't currently support in the app. That's fine, because all of this is internal currently.

I settled on this because it addresses the second two items on that previous list:

• git worktree add shares the same .git directory from the main worktree, so we don't need to reclone the repository,
• in this new worktree we can change the HEAD commit to another branch (or use as a detached HEAD) and the index/working directory state is kept separate to the main worktree

With a worktree for us to maniplate and test independent of the main repository, the rest of the flow becomes pretty straightforward:

• we checkout the base branch in the new worktree
• we run git format-patch to generate the patch series that the rebase will perform and emit this to stdout
• we then can use git apply with the patch series against the base branch to see if it can be applied cleanly

Review Plan

As this PR touches a few areas, I'm going to break them up into a series PRs to make reviews more focused. The tentative plan:

• introduce abstraction OnDiskRepository that is implemented by Repository and WorkTree - introduce OnDiskRepository type to support using worktrees in Git operations #7819
• introduce worktrees Git APIs and tests
• introduce patch format and apply Git and tests
• ???

TODO

• more testing
• caching to prevent user re-running same rebase preview
• ???
• docs

Release notes

Notes: rebase flow will now warn the user if conflicts are expected