branchmap: revert c34532365b38 for Python 2.7 compatibility

Old versions of python 2.7 don't like that the second argument to
struct.unpack_from is a bytearray, so the change removing the util.buffer
around that argument in branchmap broke running on older versions of python
2.7.

Differential Revision: https://phab.mercurial-scm.org/D330

mercurial/branchmap.py

@@ -406,7 +406,8 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, self._rbcrevs, rbcrevidx)
+        cachenode, branchidx = unpack_from(
+            _rbcrecfmt, util.buffer(self._rbcrevs), rbcrevidx)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask