conflicts: Implement `ResolveConflicts` with Git
The ResolveConflicts
RPC works by internally calling the Resolve
functionality of our Git2Go executor. To move away from Git2Go entirely
we implement the conflict.Resolve
function.
This commit utilizes this function to perform conflict resolution behind a feature flag. Eventually we can get rid of the feature flag and delete the git2go implementation code related to it.
One drawback behind using Git for conflict resolution is that Git doesn't retain terminating linefeeds in conflicted files. While we could hack around this, it is safer to be inline with Git. This drawback is noted in the tests.
Closes #4580 (closed)