feat: check if there are changes found in the git tree (knqyf263#3)

main.go

@@ -84,6 +84,15 @@ func run(c config) error {
 		return xerrors.Errorf("unable to get a worktree based on the given fs: %w", err)
 	}
 
+	s, err := w.Status()
+	if err != nil {
+		return xerrors.Errorf("unable to get the working tree status: %w", err)
+	}
+
+	if !s.IsClean() {
+		return xerrors.New("the repository is dirty: commit all changes before running 'cob'")
+	}
+
 	err = w.Reset(&git.ResetOptions{Commit: *prev, Mode: git.HardReset})
 	if err != nil {
 		return xerrors.Errorf("failed to reset the worktree to a previous commit: %w", err)