Basic validation for paths on command line

Disallow paths starting with a hyphen - those are probably due to
quoting multiple arguments like "--lint pylint". Fixes #147.

src/darker/__main__.py

@@ -278,7 +278,10 @@ def main(argv: List[str] = None) -> int:
     """
     if argv is None:
         argv = sys.argv[1:]
-    args, config, config_nondefault = parse_command_line(argv)
+    try:
+        args, config, config_nondefault = parse_command_line(argv)
+    except ArgumentError as exc_info:
+        sys.exit(exc_info)
     logging.basicConfig(level=args.log_level)
     if args.log_level == logging.INFO:
         formatter = logging.Formatter("%(levelname)s: %(message)s")

src/darker/command_line.py

@@ -1,4 +1,6 @@
-from argparse import ArgumentParser, Namespace
+"""Command line parsing for the ``darker`` binary"""
+
+from argparse import Action, ArgumentError, ArgumentParser, Namespace
 from typing import Any, List, Optional, Text, Tuple
 
 from darker import help as hlp
@@ -105,6 +107,14 @@ def parse_command_line(argv: List[str]) -> Tuple[Namespace, DarkerConfig, Darker
     #    This is used to find out differences between the effective configuration and
     #    default configuration values, and print them out in verbose mode.
     parser_with_original_defaults = make_argument_parser(require_src=True)
+    invalid_src_paths = [src for src in args.src if src.startswith("-")]
+    if invalid_src_paths:
+        invalid_path_reprs = " ".join(repr(path) for path in invalid_src_paths)
+        raise ArgumentError(
+            Action(["src"], dest="src", metavar="PATH"),
+            f"PATH can't begin with a hyphen: {invalid_path_reprs}\n"
+            "Maybe check your shell quoting?",
+        )
     return (
         args,
         get_effective_config(args),