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 `.-_`.

pre_commit_hooks/check_permitted_path_characters.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+import argparse
+import string
+from typing import Sequence
+
+DEFAULT_ALLOWLIST = string.ascii_letters + string.digits + '-_./'
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*')
+    parser.add_argument(
+        '--allowlist',
+        default=DEFAULT_ALLOWLIST,
+        help=(
+            'Override the default allowlist of permitted characters. The'
+            ' default is %(default)s'
+        ),
+    )
+    parser.add_argument(
+        '--extra-allowlist',
+        default='',
+        help='Extend the default allowlist with these characters.',
+    )
+    args = parser.parse_args(argv)
+
+    allowlist = set(args.allowlist + args.extra_allowlist)
+
+    retcode = 0
+    for filename in args.filenames:
+        # check the entire path, not just the filename, to catch directories
+        # with invalid characters
+        file_chars = set(filename)
+        if not file_chars.issubset(allowlist):
+            # sorted and stringified for readability
+            pretty_allowlist = ''.join(sorted(allowlist))
+            pretty_banlist = repr(''.join(sorted(file_chars - allowlist)))
+            print(
+                f'"{filename}" contains characters not in the allowlist:'
+                f' "{pretty_banlist}". Allowlist is: "{pretty_allowlist}".',
+            )
+            retcode = 1
+
+    return retcode
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())