Include Git branch

[muety]

wakatime-cli has functionality to automatically detect the Git branch that you're current working on, see here, here. However, the detection will only run if the heartbeat's entity is a file, see here.

For the ZSH plugin, I think it could be use useful to override that functionality by passing the current Git (or Mercurial?) branch in the current working directory (if existing) via --alternate-branch.

Brought up in the discussion at https://github.com/muety/wakapi/discussions/580#discussioncomment-7983365.