filelog: open the writing context a bit earlier in `addgroup`

This is a small change made ahead of the next patch for clarification.

Differential Revision: https://phab.mercurial-scm.org/D11269

mercurial/filelog.py

@@ -157,13 +157,14 b' class filelog(object):'
                 )
             )
 
-        return self._revlog.addgroup(
-            deltas,
-            linkmapper,
-            transaction,
-            addrevisioncb=addrevisioncb,
-            duplicaterevisioncb=duplicaterevisioncb,
-        )
+        with self._revlog._writing(transaction):
+            return self._revlog.addgroup(
+                deltas,
+                linkmapper,
+                transaction,
+                addrevisioncb=addrevisioncb,
+                duplicaterevisioncb=duplicaterevisioncb,
+            )
 
     def getstrippoint(self, minlink):
         return self._revlog.getstrippoint(minlink)