feat: git branch: read from HEAD on newly initialized repo (#1336)

* 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

1 files changed, 13 insertions, 2 deletions

diff --git a/src/context.rs b/src/context.rs
index 27feac0f7..17a73c688 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -321,8 +321,19 @@ fn get_current_branch(repository: &Repository) -> Option<String> {
         Err(e) => {
             return if e.code() == UnbornBranch {
                 // HEAD should only be an unborn branch if the repository is fresh,
-                // in that case assume "master"
-                Some(String::from("master"))
+                // in that case read directly from `.git/HEAD`
+                let mut head_path = repository.path().to_path_buf();
+                head_path.push("HEAD");
+
+                // get first line, then last path segment
+                fs::read_to_string(&head_path)
+                    .ok()?
+                    .lines()
+                    .next()?
+                    .trim()
+                    .split('/')
+                    .last()
+                    .map(|r| r.to_owned())
             } else {
                 None
             };