feat: add env::git_shell() to obtain the shell Git would be using.

This is particularly useful to execute Git hooks.

gix-path/src/env/mod.rs

@@ -28,6 +28,23 @@ pub fn installation_config_prefix() -> Option<&'static Path> {
     installation_config().map(git::config_to_base_path)
 }
 
+/// Return the shell that Git would prefer as login shell, the shell to execute Git commands from.
+///
+/// On Windows, this is the `bash.exe` bundled with it, and on Unix it's the shell specified by `SHELL`,
+/// or `None` if it is truly unspecified.
+pub fn login_shell() -> Option<&'static Path> {
+    static PATH: Lazy<Option<PathBuf>> = Lazy::new(|| {
+        if cfg!(windows) {
+            dbg!(installation_config_prefix())
+                .and_then(|p| p.parent())
+                .map(|p| p.join("usr").join("bin").join("bash.exe"))
+        } else {
+            std::env::var_os("SHELL").map(PathBuf::from)
+        }
+    });
+    PATH.as_deref()
+}
+
 /// Return the name of the Git executable to invoke it.
 /// If it's in the `PATH`, it will always be a short name.
 ///

gix-path/tests/path/env.rs

@@ -7,6 +7,13 @@ fn exe_invocation() {
     );
 }
 
+#[test]
+fn login_shell() {
+    assert!(gix_path::env::login_shell()
+        .expect("There should always be the notion of a shell used by git")
+        .exists());
+}
+
 #[test]
 fn installation_config() {
     assert_ne!(