branchmap: remove use of buffer() to support Python 2.6

The use of buffer() was added in 7359157b9e46 to support Python 2.6,
which we no longer support.

mercurial/branchmap.py

@@ -406,8 +406,7 b' class revbranchcache(object):'
 
         # fast path: extract data from cache, use it if node is matching
         reponode = changelog.node(rev)[:_rbcnodelen]
-        cachenode, branchidx = unpack_from(
-            _rbcrecfmt, util.buffer(self._rbcrevs), rbcrevidx)
+        cachenode, branchidx = unpack_from(_rbcrecfmt, self._rbcrevs, rbcrevidx)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask