Add check for permitted characters in pathnames

Add a new checker that checks for allowed characters in pathnames.

Includes arguments for overriding the default allowed list and extending
it.

Default allow list is alphanumeric plus `.-_`.

tests/check_permitted_path_characters_test.py

@@ -0,0 +1,50 @@
+from __future__ import annotations
+
+from pre_commit_hooks.check_permitted_path_characters import main
+
+
+def test_main_all_pass():
+    ret = main(
+        [
+            '/some/path/foo_test.py',
+            './relative/path/bar_test.py',
+            'filename-only.py',
+        ],
+    )
+    assert ret == 0
+
+
+def test_main_default_chars():
+    # use '--' for separating pathnames from args, so pathnames with leading
+    # '-' are not interpreted as flags
+    ret = main(
+        [
+            '--',
+            '-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_',
+            'abcdefghijklmnopqrstuvwxyz',
+        ],
+    )
+    assert ret == 0
+
+    ret = main(['+'])
+    assert ret == 1
+
+
+def test_main_invalid_dir():
+    ret = main(['--', '/some+funky%%dir/pathname'])
+    assert ret == 1
+
+
+def test_main_allowlist():
+    ret = main(['--allowlist', 'abc', 'invalid.py'])
+    assert ret == 1
+    ret = main(['--allowlist', 'abc', 'cba'])
+    assert ret == 0
+    # a pathological case
+    ret = main(['--allowlist', '\b\x01\t/.', '\b.\x01/\t'])
+    assert ret == 0
+
+
+def test_main_extra_allowlist():
+    ret = main(['--extra-allowlist', '+', 'valid+'])
+    assert ret == 0