gitstats: Optimization in the 'belongs to' metric

gitstats: Optimization in the 'belongs to' metric

Retreive the parentage information in 'one big go' instead
of on a 'per branch' basis. This is now possible since we
do not need the individual rev-lists anymore since the
previous commit. This has two advantages, the rev-lists
mechanism cuts down the amount of parentage-information we
need to parse by not listing doubles (for example, we
always get the parentage information of the first commit
multiple times when there are multiple branches that branch
off from it). A second advantage is that this saves us a
lot of calls to the git binary (1 call, instead of one for
each branch).