Fix wrong GitCommandNotFound command from refresh

See #1809. The test extended in the previous commit now passes.

Author: EliahKagan

Diff for: git/cmd.py

@@ -483,7 +483,7 @@ def refresh(cls, path: Union[None, PathLike] = None) -> bool:
             else:
                 # After the first refresh (when GIT_PYTHON_GIT_EXECUTABLE is no longer
                 # None) we raise an exception.
-                raise GitCommandNotFound("git", err)
+                raise GitCommandNotFound(new_git, err)
 
         return has_git