Implement merges using the new 'git merge-tree' command

A new MergeGitMergeTree feature flag is created. Under this feature flag, there is a new implementation of merge using the new --write-tree mode for git merge-tree developed by Elijah Newren.

In be733e120016a076926da3491c0c77d439686933 the en/merge-tree branch was merged into Git's master branch, so the new --write-tree mode for git merge-tree will be available in Git v2.38.0 which should be released at the beginning of October 2022.

This MR adds a function to use git-merge-tree(1) to perform the in memory merge behind a feature flag.

fixes: #4154 (closed)