feat(git): honor GIT_DIR environment variable (#1348)

src/context.rs

@@ -136,7 +136,11 @@ impl<'a> Context<'a> {
     pub fn get_repo(&self) -> Result<&Repo, std::io::Error> {
         self.repo
             .get_or_try_init(|| -> Result<Repo, std::io::Error> {
-                let repository = Repository::discover(&self.current_dir).ok();
+                let repository = if env::var("GIT_DIR").is_ok() {
+                    Repository::open_from_env().ok()
+                } else {
+                    Repository::discover(&self.current_dir).ok()
+                };
                 let branch = repository
                     .as_ref()
                     .and_then(|repo| get_current_branch(repo));

tests/testsuite/git_branch.rs

@@ -1,6 +1,7 @@
 use ansi_term::Color;
 use remove_dir_all::remove_dir_all;
 use std::io;
+use std::path::Path;
 use std::process::Command;
 
 use crate::common::{self, TestCommand};
@@ -122,6 +123,29 @@ fn test_works_with_unborn_master() -> io::Result<()> {
     remove_dir_all(repo_dir)
 }
 
+#[test]
+fn test_git_dir_env_variable() -> io::Result<()> {
+    let repo_dir = tempfile::tempdir()?.into_path();
+
+    Command::new("git")
+        .args(&["init"])
+        .current_dir(&repo_dir)
+        .output()?;
+
+    let output = common::render_module("git_branch")
+        .env("GIT_DIR", Path::new(&repo_dir).join(".git"))
+        .output()
+        .unwrap();
+    let actual = String::from_utf8(output.stdout).unwrap();
+
+    let expected = format!(
+        "on {} ",
+        Color::Purple.bold().paint(format!("\u{e0a0} {}", "master")),
+    );
+    assert_eq!(expected, actual);
+    remove_dir_all(repo_dir)
+}
+
 fn test_truncate_length(
     branch_name: &str,
     truncate_length: i64,