revlog: close the index file handle after the data one

This make sure the data file is flushed before the index. preventing the index
to reference unflushed data.

Differential Revision: https://phab.mercurial-scm.org/D10776

mercurial/revlog.py

@@ -2267,10 +2267,12 b' class revlog(object):'
                     self._write_docket(transaction)
             finally:
                 self._writinghandles = None
+                if dfh is not None:
+                    dfh.close()
+                # closing the index file last to avoid exposing referent to
+                # potential unflushed data content.
                 if ifh is not None:
                     ifh.close()
-                if dfh is not None:
-                    dfh.close()
 
     def _write_docket(self, transaction):
         """write the current docket on disk