* Git branch: read from HEAD on newly init repo
On a newly initialized git repo, there are no branches created until a
commit is made. Previously, starship handled this by having a default
branch "master" for when branch `head` could not be read.
However, if a user wants to set a different default branch name, that
name won't appear on starship until a commit is made to the branch.
If git2 provides a way to read the default branch name, we can use that,
but at the moment it's not obvious how.
For the moment, we can directly read `.git/HEAD`, which contains the
name of the default branch head reference. This commit implements this
strategy.
Closes#1327
* update git_branch test from unborn master to unborn default
* cargo fmt
489838e6a2