Prefer younger merge bases over older ones.

git-core prefers younger merge bases over older ones in case that multiple valid merge bases exists.

I'm not sure whether `git_commit_list_insert_by_date` is the correct place to make this change, but this function is only ever called by `git_merge__bases_many`, so I hope that's ok.

I can also prepare a real test case for this, if the location of this change is determined to be ok. :smile: