[PATCH 7 of 9 py3] branchmap: don't use buffer() on Python 3

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <augie at google.com>
# Date 1489297759 18000
#      Sun Mar 12 00:49:19 2017 -0500
# Node ID 062d30be868aad888f8d2e51d65999cb8c94b81a
# Parent  b9d20b6e86e011db2a11abccc26250c7a83e8aac
branchmap: don't use buffer() on Python 3

This is certainly slower than the Python 2 code, but it works, and we
can revisit it later if it's a problem.

diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -19,6 +19,7 @@ from .node import (
 from . import (
     encoding,
     error,
+    pycompat,
     scmutil,
     util,
 )
@@ -408,8 +409,11 @@ class revbranchcache(object):
 
         # fast path: extract data from cache, use it if node is matching
         reponode = changelog.node(rev)[:_rbcnodelen]
-        cachenode, branchidx = unpack(
-            _rbcrecfmt, buffer(self._rbcrevs, rbcrevidx, _rbcrecsize))
+        if pycompat.ispy3:
+            data = self._rbcrevs[rbcrevidx:rbcrevidx + _rbcrecsize]
+        else:
+            data = buffer(self._rbcrevs, rbcrevidx, _rbcrecsize)
+        cachenode, branchidx = unpack(_rbcrecfmt, data)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask