[merge] update to external diff fixes

[John Arbash Meinel]

The attached patch is to close bug 56307.
https://launchpad.net/products/bzr/+bug/56307

Basically, now that we explicitly test that we can spawn diff and handle
binary files, we have to also support when LANG is not english.
So now we spawn diff with LANG=C set, and if it returns a bad error code
(retcode=2), we run it again with native LANG and report the error.

It includes a test that when using an external diff we report what diff
would naturally report. And I had Martin Pitt run it on his German
language Edgy machine, and it worked. (Dapper doesn't seem to have i18n
for diff)

John
=:->
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