gitstats: Made the 'belongs to' metric recursive

gitstats: Made the 'belongs to' metric recursive

Note: next time, try the recursive approach first, it works
like a charm! The big townside here is that it comes at the
cost of having to manually walk the entire tree, this is
very expensive, especially on windows this will be
impossibly slow for large repositories. (This is mainly due
to the high amount of calls to 'git log --pretty=format:%P'
which requires a fork each time. An optimization would be
to first get all the parents, and then query a hashtable.