tests/test-issue842

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.

#!/bin/sh

hg init test
cd test
echo foo > a
hg ci -Ama

hg up -r0000
echo bar > a
echo % should issue warning
hg ci -Amb

hg up -r0000
echo stuffy > a
echo % should not issue warning
hg ci -q -Amc

hg up -r0000
echo crap > a
hg branch testing
echo % should not issue warning
hg ci -q -Amd