revset: check for collisions between alias argument names in the declaration

Before this patch, collisions between alias argument names in the
declaration are ignored, and this silently causes unexpected alias
evaluation.

This patch checks for such collisions, and aborts (or shows a warning) when
collisions are detected.

This patch doesn't add a test to "test-revset.t", because a doctest is
enough to test the collisions detection itself.

mercurial/revset.py

@@ -2203,6 +2203,8 b' def _parsealiasdecl(decl):'
     ('foo($1, $2', None, None, 'at 10: unexpected token: end')
     >>> _parsealiasdecl('foo("string')
     ('foo("string', None, None, 'at 5: unterminated string')
+    >>> _parsealiasdecl('foo($1, $2, $1)')
+    ('foo', None, None, 'argument names collide with each other')
     """
     p = parser.parser(_tokenizealias, elements)
     try:
@@ -2227,6 +2229,9 b' def _parsealiasdecl(decl):'
                 if not isvalidsymbol(arg):
                     return (decl, None, None, _("invalid argument list"))
                 args.append(getsymbol(arg))
+            if len(args) != len(set(args)):
+                return (name, None, None,
+                        _("argument names collide with each other"))
             return (name, ('func', ('symbol', name)), args, None)
 
         return (decl, None, None, _("invalid format"))