conflicts: Introduce and use `conflictFilesWithGitMergeTree`
The current implementation of ListConflictFiles
uses Git2Go to find
the file conflicts, parses this data and returns the information to the
client.
As part of the plan to move away from Git2Go, we introduce a new
function conflictFilesWithGitMergeTree
which uses git-merge-tree(1) to
find the conflicted files. We then use this implementation behind the
ListConflictFilesMergeTree
featureflag.
Closes #4576 (closed)