prep: add md_* properly/catch exception when no md_* exists

Author: Gerit Wagner

colrev/ops/prep.py

@@ -35,6 +35,7 @@
 from colrev.package_manager.package_manager import PackageManager
 from colrev.writer.write_utils import to_string
 from colrev.writer.write_utils import write_file
+import git
 
 if typing.TYPE_CHECKING:  # pragma: no cover
     import colrev.package_manager.package_base_classes as base_classes
@@ -932,7 +933,10 @@ def _create_prep_commit(
             {r[Fields.ID]: r for r in prepared_records}, partial=True
         )
         self._log_commit_details(prepared_records)
-        self.review_manager.dataset.git_repo.add_changes("data/search/md_*")
+        try:
+            self.review_manager.dataset.git_repo.repo.git.add("data/search/md_*")
+        except git.GitCommandError:
+            pass
         self.review_manager.create_commit(
             msg="Prep: improve record metadata",
         )