Annotate safer_popen broad enough for all platforms

This fixes another static typing error reported by mypy.

(The annotation could be made more specific in the future by making
a custom protocol for it, which may or may not be worthwhile, given
that `**kwargs: Any` would still have to be present after whatever
typed keyword arguments the protocol's `__call__` method listed,
since some callers intentionally forward arbitrary extra keyword
arguments through safer_popen to Popen.)

git/cmd.py

@@ -278,6 +278,8 @@ def _safer_popen_windows(
         )
 
 
+safer_popen: Callable[..., Popen]
+
 if os.name == "nt":
     safer_popen = _safer_popen_windows
 else: