make branch grouping more intelligent

[psimk]

Currently branches are grouped by the author of the latest ahead commit.
This means that as long as someone makes the latest ahead commit, then they will have "ownership" of the branch on the spreadsheet. I hope I don't have to explain, why this is sub-optimal.

The "ownership" of a branch should be detected based on:

• how many commits an author has made to the branch.
• if there are two authors of a branch and their commit count matches, then the author who made the first commit is selected.