revlog: use context manager for index file life time in __init__

This is clearer, safer and more modern.

mercurial/revlog.py

@@ -621,13 +621,12 class revlog(object):
         indexdata = ''
         self._initempty = True
         try:
-            f = self._indexfp()
+            with self._indexfp() as f:
             if (mmapindexthreshold is not None and
                     self.opener.fstat(f).st_size >= mmapindexthreshold):
                 indexdata = util.buffer(util.mmapread(f))
             else:
                 indexdata = f.read()
-            f.close()
             if len(indexdata) > 0:
                 v = versionformat_unpack(indexdata[:4])[0]
                 self._initempty = False