tests/testlib/crash_transaction_late.py

files: speed up `hg files` when no flags change display

It's not the first time I see slowness from this command slow down
tools built on top of hg.

The majority of the time is spent merely printing the result before
this change, which is clearly not how it should be (especially since
the computation of the result also looks slow).

Running `hg files` in mozilla-central:

parent revision: 1,260s
this commit: 0,683s
this commit without batching ui.write: 0,931s
this commit replacing the body of the loop with `pass`: 0,566s

This looks like a prime candidate for a rust fast path, but until
then, it seems reasonable to optimize the python.

Differential Revision: https://phab.mercurial-scm.org/D8586

# tiny extension to abort a transaction very late during test
#
# Copyright 2020 Pierre-Yves David <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

from mercurial import (
    error,
    transaction,
)


def abort(fp):
    raise error.Abort(b"This is a late abort")


def reposetup(ui, repo):

    transaction.postfinalizegenerators.add(b'late-abort')

    class LateAbortRepo(repo.__class__):
        def transaction(self, *args, **kwargs):
            tr = super(LateAbortRepo, self).transaction(*args, **kwargs)
            tr.addfilegenerator(
                b'late-abort', [b'late-abort'], abort, order=9999999
            )
            return tr

    repo.__class__ = LateAbortRepo