Closes #8049

Description

Here's little bonus PR at the tail end of the Git large output work. I've been eyeing our determineMergeability function because it's the only remaining function which uses spawnGit. I was pleasantly surprised to find that in the time that's passed since we initially wrote determineMergeability there's been some great improvements to git-merge-tree.

Most importantly that it's able to perform "real merges" (--write-tree), i.e. the exact same merge that would occur if you ran git merge in the terminal except that it won't touch the index or the working directory. This is what we've been looking for to resolve #8049.

The merge will be done in memory and the resulting tree will be written to disk but unless a merge occurs it will be dangling and cleaned up on the next garbage collection.

And thanks to --name-only and -z we can get rid of all logic to parse diffs looking for conflicts markers and let Git do the work for us 🎉

I also removed our promiseWithTimeout "hack" and added an asynchronous cache (adding the p-memoization library instead of rolling our own) which will ensure we don't have to recalculate the merge tree when the user clicks around in the merge dialog.

Screenshots

Release notes

Notes: