Make HIDE_* attributes always bool

For now, this doesn't change how the correspondng environment
variables are interpreted, in terms of truth and falsehood.
But it does *convert* them to bool, so that the values of the
HIDE_WINDOWS_KNOWN_ERRORS and HIDE_WINDOWS_FREEZE_ERRORS
attributes are always bools.

It also updates the tests accordingly, to validate this behavior.

git/util.py

@@ -110,7 +110,7 @@
 log = logging.getLogger(__name__)
 
 
-def _read_env_flag(name: str, default: bool) -> Union[bool, str]:
+def _read_env_flag(name: str, default: bool) -> bool:
     try:
         value = os.environ[name]
     except KeyError:
@@ -121,9 +121,8 @@ def _read_env_flag(name: str, default: bool) -> Union[bool, str]:
         name,
     )
 
-    # FIXME: This should always return bool, as that is how it is used.
     # FIXME: This should treat some values besides "" as expressing falsehood.
-    return value
+    return bool(value)
 
 
 #: We need an easy way to see if Appveyor TCs start failing,

test/test_util.py

@@ -520,7 +520,7 @@ def run_parse(value):
             )
             return ast.literal_eval(output)
 
-        assert_true_iff_win = self.assertTrue if os.name == "nt" else self.assertFalse
+        true_iff_win = os.name == "nt"  # Same as is_win, but don't depend on that here.
 
         truthy_cases = [
             ("unset", None),
@@ -542,8 +542,8 @@ def run_parse(value):
 
         for msg, env_var_value in truthy_cases:
             with self.subTest(msg, env_var_value=env_var_value):
-                assert_true_iff_win(run_parse(env_var_value))
+                self.assertIs(run_parse(env_var_value), true_iff_win)
 
         for msg, env_var_value in falsy_cases:
             with self.subTest(msg, env_var_value=env_var_value):
-                self.assertFalse(run_parse(env_var_value))
+                self.assertIs(run_parse(env_var_value), False)