Prevent endless loop by checking result of filepath.Dir

filepath.Dir will return the same result when you're calling it on the
root directory of your filesystem. Checking when this happens is
required so that you break out of your loop.

This code used to loop endlessly when called outside of a git repo.
This commit fixes that.

internal/cmd/stack/open_command.go

@@ -122,6 +122,13 @@ func getGitRepositorySubdir() (string, error) {
 		} else if !os.IsNotExist(err) {
 			return "", fmt.Errorf("couldn't stat .git directory: %w", err)
 		}
+
+		if newRoot := filepath.Dir(root); newRoot != root {
+			root = newRoot
+		} else {
+			return "", fmt.Errorf("couldn't find .git directory in %s or any of its parents", current)
+		}
+
 		root = filepath.Dir(root)
 	}