tests/failfilemerge.py

simplemerge: move default labels to simplemerge extension

I want to remove knowledge of paths from the `simplemerge` core
module. As part of that, let's move the default labels from the core
module to the extension.

Differential Revision: https://phab.mercurial-scm.org/D11977

# extension to emulate interrupting filemerge._filemerge

from __future__ import absolute_import

from mercurial import (
    error,
    extensions,
    filemerge,
)


def failfilemerge(*args, **kwargs):
    raise error.Abort(b"^C")


def extsetup(ui):
    extensions.wrapfunction(filemerge, 'filemerge', failfilemerge)