gitstats: Bugfix to only ignore if dilution was 'worse'

gitstats: Bugfix to only ignore if dilution was 'worse'

In the case that a commit has already been seen, it should
also be checked if the dilution that is was seen by was
lower. If this is not checked, the result is that when a
'bad' path is walked first, later on when trying the
'better' path, it will be aborted due to the fact that this
commit was already examined. A simple solution is to store
by which dilution the commit was seen in a dictionary.
This also fixes the t8101-metrics test, the order of the
output had changed, but the test wasn't updated to reflect
this change.

Bug-introduced-in: 556fe5b883c97ad5ea8143d987d3799673e17001