Enable diff3 for conflict resolution
Problem to solve
Merge conflicts are painful to resolve. GitLab offers a merge conflict resolution tool in the UI, but it's often not clear whether we should take "our" or "their" changes, or some combination of the two, when using it.
Intended users
Further details
Git includes a diff3 mode for conflict resolution that tries to solve exactly this problem - it shows additional context about the ancestor commits, making it easier to pick a side or combination of changes, especially for the simple conflicts we allow resolution of in the UI.
http://psung.blogspot.com/2011/02/reducing-merge-headaches-git-meets.html
https://blog.nilbus.com/take-the-pain-out-of-git-conflict-resolution-use-diff3/
Proposal
Enable diff3 mode for our conflict resolution UI. I'd have no qualms about making this unconditional, but we could consider allowing the user the select the merge mode when they invoke the UI, if diff3 is unpopular for reasons I'm unaware of.