parser: resolve ambiguity where both prefix and primary actions are defined

If both actions are defined, a primary-expression action is accepted only if
the next token never starts new term. For example,

  parsed as primary expression:
  ":"   # next token 'end' has no action
  "(:)" # next token ')' has no action
  ":+y" # next token '+' is infix operator

  parsed as prefix operator:
  ":y"  # next token 'y' is primary expression
  ":-y" # next token '-' is prefix operator

This is mostly the same resolution as the infix/suffix rules.

mercurial/parser.py

@@ -49,9 +49,9 b' class parser(object):'
         return expr
     def _parse(self, bind=0):
         token, value, pos = self._advance()
-        # handle prefix rules on current token
+        # handle prefix rules on current token, take as primary if unambiguous
         primary, prefix = self._elements[token][1:3]
-        if primary:
+        if primary and not (prefix and self._hasnewterm()):
             expr = (primary, value)
         elif prefix:
             expr = (prefix[0], self._parseoperand(*prefix[1:]))