Treat false-seeming HIDE_* env var values as false

This changes how HIDE_WINDOWS_KNOWN_ERRORS and
HIDE_WINDOWS_FREEZE_ERRORS environment variables, if present, are
interpreted, so that values that strongly seem intuitivley to
represent falsehood now do.

Before, only the empty string was treated as false. Now:

- "0", "false", "no", and their case variants, as well as the empty
  string, are treated as false.

- The presence of leading and trailing whitespace in the value now
  longer changes the truth value it represents. For example,
  all-whitespace (e.g., a space) is treated as false.

- Values other than the above false values, and the recognized true
  values "1", "true", "yes", and their variants, are still treated
  as true, but issue a warning about how they are unrecognied.

git/util.py

@@ -121,8 +121,14 @@ def _read_env_flag(name: str, default: bool) -> bool:
         name,
     )
 
-    # FIXME: This should treat some values besides "" as expressing falsehood.
-    return bool(value)
+    adjusted_value = value.strip().lower()
+
+    if adjusted_value in {"", "0", "false", "no"}:
+        return False
+    if adjusted_value in {"1", "true", "yes"}:
+        return True
+    log.warning("%s has unrecognized value %r, treating as %r.", name, value, default)
+    return default
 
 
 #: We need an easy way to see if Appveyor TCs start failing,

test/test_util.py

@@ -529,15 +529,15 @@ def run_parse(value):
             ("true-seeming", "True"),
             ("true-seeming", "yes"),
             ("true-seeming", "YES"),
+        ]
+        falsy_cases = [
+            ("empty", ""),
+            ("whitespace", " "),
             ("false-seeming", "0"),
             ("false-seeming", "false"),
             ("false-seeming", "False"),
             ("false-seeming", "no"),
             ("false-seeming", "NO"),
-            ("whitespace", " "),
-        ]
-        falsy_cases = [
-            ("empty", ""),
         ]
 
         for msg, env_var_value in truthy_cases: