localrepo: Allow unrelated histories in merge tree
In Git, by default git-merge(1) and git-merge_tree(1) throw an error when an attempted merge is made on two commits that do not have a shared ancestor. There are times when we want to allow this, so create an option that allows this.
The enforcement of unrelated histories led to some merge requests failing in production: #4600 (closed)
Edited by John Cai