Fix a bug where a request for help wasn't detected.

Testing Done:
options unittests pass.

Reviewed at https://rbcommons.com/s/twitter/r/1467/

src/python/pants/option/arg_splitter.py

@@ -45,6 +45,8 @@ class ArgSplitter(object):
 
   Handles help flags (-h, --help and the scope 'help') specially.
   """
+  _HELP_FLAGS = ('-h', '--help')
+
   def __init__(self, known_scopes):
     self._known_scopes = set(known_scopes + ['help'])
     self._unconsumed_args = []  # In reverse order, for efficient popping off the end.
@@ -115,9 +117,11 @@ def assign_flag_to_scope(flag, default_scope):
     while self._unconsumed_args and not self._at_double_dash():
       arg = self._unconsumed_args.pop()
       if arg.startswith(b'-'):
-        # During migration we allow flags here, and assume they are in global scope.
-        # TODO(benjy): Should we allow this even after migration?
-        assign_flag_to_scope(arg, GLOBAL_SCOPE)
+        # We assume any args here are in global scope.
+        if arg in self._HELP_FLAGS:
+          self._is_help = True
+        else:
+          assign_flag_to_scope(arg, GLOBAL_SCOPE)
       else:
         targets.append(arg)
 
@@ -154,7 +158,7 @@ def _consume_flags(self):
     flags = []
     while self._at_flag():
       flag = self._unconsumed_args.pop()
-      if flag in ('-h', '--help'):
+      if flag in self._HELP_FLAGS:
         self._is_help = True
       else:
         flags.append(flag)

tests/python/pants_test/option/test_arg_splitter.py

@@ -111,3 +111,5 @@ def test_help_detection(self):
                 {'': [], 'compile': []}, [], [], expected_is_help=True)
     self._split('./pants compile --help test', ['compile', 'test'],
                 {'': [], 'compile': [], 'test': []}, [], [], expected_is_help=True)
+    self._split('./pants test src/foo/bar:baz -h', ['test'],
+                {'': [], 'test': []}, ['src/foo/bar:baz'], [], expected_is_help=True)