phases: gather the logic for phasesets update in a single method
This logic is duplicated around for no good reason, we gather it in a single
place.
The conditional is the new function are a bit weird as we about going to extend it soon.
# extension to emulate interrupting filemerge._filemerge
from mercurial import (
error,
extensions,
filemerge,
)
def failfilemerge(*args, **kwargs):
raise error.Abort(b"^C")
def extsetup(ui):
extensions.wrapfunction(filemerge, 'filemerge', failfilemerge)