[git] Sort branches by reverse commit date
[git] Sort branches by reverse commit date

This config option is new in [git 2.19].

h/t gabebw/dotfiles@dfda835

> The author date (`authordate` key) on a commit is preserved on rebase
> / cherry-pick etc, but the commit date (`committerdate` key) is
> changed.

[git 2.19]:https://blog.github.com/2018-09-10-highlights-from-git-2-19/#sorting-branches

reshleman/dotfiles,reshleman/dotfiles