SuperSize: Fix "supersize diff" not truncating long tail of small symbols

This was my intention in b0e391b3, but
the logic was being skipped for diffs!

Change-Id: Ibf83dbc3274612027f618782227d482b72c1fb44
Reviewed-on: https://chromium-review.googlesource.com/1226293Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591355}

tools/binary_size/libsupersize/models.py

@@ -873,7 +873,7 @@ class SymbolGroup(BaseSymbol):
     """
     return self._CreateTransformed(
         self._filtered_symbols, filtered_symbols=self._symbols,
-        section_name=SECTION_MULTIPLE, is_default_sorted=False)
+        section_name=SECTION_MULTIPLE)
 
   def GroupedBy(self, func, min_count=0, group_factory=None):
     """Returns a SymbolGroup of SymbolGroups, indexed by |func|.