crucible.py should disable customized svn diff

It is not uncommon to have something like

[helpers]
diff-cmd = colordiff

in ~/.subversion/config and that breaks patch submission with crucible.py .

To disable such a customization, crucible.py should do something like

check_output([self.configuration.executables[self.executable()], 'diff', '--internal-diff', '-x', '-u'])