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

tests/testsuite/git_branch.rs

@@ -100,7 +100,7 @@ fn test_japanese_truncation() -> io::Result<()> {
 }
 
 #[test]
-fn test_works_with_unborn_master() -> io::Result<()> {
+fn test_works_with_unborn_default_branch() -> io::Result<()> {
     let repo_dir = tempfile::tempdir()?.into_path();
 
     Command::new("git")
@@ -108,6 +108,11 @@ fn test_works_with_unborn_master() -> io::Result<()> {
         .current_dir(&repo_dir)
         .output()?;
 
+    Command::new("git")
+        .args(&["symbolic-ref", "HEAD", "refs/heads/main"])
+        .current_dir(&repo_dir)
+        .output()?;
+
     let output = common::render_module("git_branch")
         .arg("--path")
         .arg(&repo_dir)
@@ -117,7 +122,7 @@ fn test_works_with_unborn_master() -> io::Result<()> {
 
     let expected = format!(
         "on {} ",
-        Color::Purple.bold().paint(format!("\u{e0a0} {}", "master")),
+        Color::Purple.bold().paint(format!("\u{e0a0} {}", "main")),
     );
     assert_eq!(expected, actual);
     remove_dir_all(repo_dir)