formatter: look for template symbols from the associated name

Otherwise symbolsused() would fail if a named template is specified with -T.

mercurial/formatter.py

@@ -420,7 +420,7 b' class templateformatter(baseformatter):'
 
     @util.propertycache
     def _symbolsused(self):
-        return self._t.symbolsuseddefault()
+        return self._t.symbolsused(self._tref)
 
     def contexthint(self, datafields):
         '''set of context object keys to be required by the template, given

tests/test-annotate.t

@@ -80,6 +80,9 b' log-like templating'
 
   $ hg annotate -T'{lines % "{rev}:{line_number}: {line}"}' a
   0:1: a
+  $ hg annotate -Ta a \
+  > --config templates.a='"{lines % "{rev}:{line_number}: {line}"}"'
+  0:1: a
 
   $ cat <<EOF >>a
   > a