cmd/git(fix[rev_list]): Honor date-range filters

why: rev_list's since/after/until/before/max_age/min_age flags were
never applied -- the guard tested the imported `datetime` module
instead of the loop value, so the condition was always False.

what:
- Test the loop value (datetime_kwarg), matching the adjacent flag
  loop, so the date-range flags reach git rev-list.

Author: tony

src/libvcs/cmd/git.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import dataclasses
-import datetime
 import os
 import pathlib
 import re
@@ -2207,7 +2206,7 @@ def rev_list(
             (max_age, "--max-age"),
             (min_age, "--min-age"),
         ]:
-            if datetime_kwarg is not None and isinstance(datetime, str):
+            if datetime_kwarg is not None and isinstance(datetime_kwarg, str):
                 local_flags.extend([datetime_shell_flag, datetime_kwarg])
 
         for int_flag, int_shell_flag in [