firefoxtree: use kwargs instead of args for pre 4.6 function signatur…

…e (Bug 1505162)

Template keywords in Mercurial 4.6+ have a new signature and API.
When upgrading the signature backwards compatibility was required
with versions 4.4 and 4.5, so the arguments were collected in
`*args` and expanded after determining which API version is
required. It appears that the arguments were actually keyword
arguments, so `*args` is actually empty and we need to get
the arguments from `kwargs`. The behaviour in question can
be demonstrated with the following example:

  Python 2.7.13 (default, Nov  6 2018, 15:52:08)
  [GCC 7.3.0] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> def a(test):
  ...     print test
  ...
  >>> r = {'test': 'hello'}
  >>> a(**r)
  hello

Despite `test` being a positional argument in `a`, we can pass a
dict with a "test" key and the function call will execute correctly.

This commit changes the function to resolve it's arguments from
`kwargs` instead of `args` on Mercurial less than 4.6.

hgext/firefoxtree/__init__.py

@@ -606,7 +606,9 @@ def template_fxheads(*args, **kwargs):
         ctx = context.resource(mapping, 'ctx')
         cache = context.resource(mapping, 'cache')
     else:
-        repo, ctx, templ, cache = args
+        repo = kwargs['repo']
+        ctx = kwargs['ctx']
+        cache = kwargs['cache']
 
     labels = _getcachedlabels(repo, ctx, cache)
     if not labels: