gitstats: Optimion to the 'belongs to' metric

gitstats: Optimion to the 'belongs to' metric

Introduce a global 'memory' that allows for early
elmination of branches that will not end up to be the best
fit anyway. By remembering if a commit was already seen in
other branches with a lower dilution, we can stop
traversing the current path. We cannot replace the normal
memory with this global memory however, since that would
prevent us from finding all branches when a commit belongs
most to multiple branches.