This guards against regressions from the changes that follow. We're
about to add a mechanism that keeps the selection anchored by commit
hash, but we need to make sure that it doesn't take effect here; after a
merge we want to select the newly added merge commit. In the current
state of the code this happens to work because we keep the selection
index the same, which happened to be 0 here; later we will change this
to explicitly select the head commit after the merge.
Stefan Haller
02be5e74ed