This code is work-in-progress, and doesn't work on a repo that has a
branched history. The issue here is that after handling a merge
commit, keeping track of which commit to look at next is non-trivial.
This solution tries to isuse a "skip" command on the walker, but this
can accidentally skip commits, when two merges have happened in
succession (maybe a bug with the impl, not the concept).
But also, the actual merge commit seems to already be part of the
norma history? So maybe we can ommit the merge commit explicitly, and
simply return a new branch handle instead.
Katharina Fey