cmd/git(refactor[typing]): Type reftag as str

why: reftag is passed to git CLI as a ref name or URL.
what:
- Narrow fetch() and pull() reftag to str | None

Author: tony

src/libvcs/cmd/git.py

@@ -420,7 +420,7 @@ def clone(
     def fetch(
         self,
         *,
-        reftag: t.Any | None = None,
+        reftag: str | None = None,
         deepen: str | None = None,
         depth: str | None = None,
         upload_pack: str | None = None,
@@ -788,7 +788,7 @@ def rebase(
     def pull(
         self,
         *,
-        reftag: t.Any | None = None,
+        reftag: str | None = None,
         repository: str | None = None,
         deepen: str | None = None,
         depth: str | None = None,