util: improved the check for the existence of the 'buffer' builtin

2to3 is unable to translate '__builtin__' calls to 'builtins' when
hasattr is used (as in hasattr(__builtin__, buffer)). Translating the
check to the format

try:
    whatever
except NameError
    # define whatever
    __builtin__.whatever = whatever

is a correct way of checking for the name and has the benefit of being
translated by 2to3. This patch implements the same idea for the
aforementioned example.

mercurial/util.py

@@ -40,7 +40,9 b' import __builtin__'
 
 def fakebuffer(sliceable, offset=0):
     return sliceable[offset:]
-if not hasattr(__builtin__, 'buffer'):
+try:
+    buffer
+except NameError:
     __builtin__.buffer = fakebuffer
 
 import subprocess