revup handles and can fix conflicts

[jerry-skydio]

We'd like to be able to handle conflicts better in a few situations instead of erroring out

• in upload, print the file paths and conflict contents
• in amend and restack, give the user an option to fix conflicts. unclear what this will look like since we can't touch the local workspace, one option is resolving conflicts in a tmpdir. however the user will be missing "git status", "git diff", "git reset" and other useful tools, so other options are to either make a sparse worktree, or to make some other "fake" git dir

[jerry-skydio]

More thoughts, although not totally fleshed out:

Given that you're cherry-picking commit A onto commit B, and get conflicts

• Identify the set of files that have conflicts, including add/delete conflicts. This could be hard because rename detection can result in different files being modified than the paths in the original patch
• Create a new commit B' on top of B that contains only the above files. If you diffed this with B it would appear as though all other files in the repo were deleted
• Create a new commit A' that also only contains those files.
• Check out a git worktree to B', then try to cherry-pick A'
• Prompt the user to manually resolve any conflicts, and either add or commit the result into A''
• For files with conflicts, graft the patch B'->A'' into the original patch
• Apply the new conflict free patch

we'd naturally have to provide revup continue, revup abort commands. users can only have one revup managed conflict tree at a time

[jerry-skydio]

https://github.com/gitgitgadget/git/pull/1122 is a huge boon to this feature. we can ff to the latest git version (2.38 once released), largely abandon the old "git apply" engine, and switch to merge-tree. In addition to much better conflict handling, this will also be slightly faster