CONTRIBUTORS

tests: add test extension implementing custom filelog storage

In order to better support partial clones, we'll need alternate
repository storage mechanisms that aren't based on revlogs.

Today, the interface for repository storage isn't very well defined.
And there are various layering violations and assumptions made
throughout the code that storage is backed by revlogs.

In order to support alternate storage mechanisms, we'll need to
formally declare and adhere to interfaces for storage. This will
be a long, arduous process.

This commit creates an extension that implements non-revlog storage
for files. It defines a custom type that quacks like the existing
revlog/filelog API but isn't backed by a revlog. The backing storage
is - for simplicity reasons - a CBOR index and per-node files
representing fulltext data.

The localrepository class is modified so file(f) returns instances of
this class instead of filelog instances.

The purpose of this extension is to tease out what the actual filelog
interface is - based on running the test harness - so we can formalize
that interface and then implement a *real* alternate storage backend.

Using `run-tests.py --extra-config-opt` to run the test harness
with this extension enabled yields 83 failures out of 634 ran
tests.

The most common test failures are due to:

* Issues with `hg verify`
* LFS and largefiles (probably flags processing related)
* Narrow.
* Any test touching or inspecting individual filelog paths.
* help and error output that is confused by the presence of an
  extension.
* `hg debug*` commands doing low-level, revlog-y things.

An 88% pass rate is pretty good for an initial implementation if you
ask me!

There is a bit of duplicate code in the new extension. That's by
design: a point of this code is to tease out dependencies on revlog.
That being said, there is opportunity to consolidate code by moving
things out of the revlog API. For example, DAG traversal operations
don't necessarily need to be implemented at the storage level. (Although
for performance reasons they probably do.) Once we have a more
well-defined interface, we could probably define the default
implementations in terms of the base interface, pull those in via
class inheritance, and have implementations override with faster
versions if they so choose. (Or something like that.) But for now,
the duplicate code should be acceptable.

Differential Revision: https://phab.mercurial-scm.org/D3029

[This file is here for historical purposes, all recent contributors
should appear in the changelog directly]

Andrea Arcangeli <andrea at suse.de>
Thomas Arendsen Hein <thomas at intevation.de>
Goffredo Baroncelli <kreijack at libero.it>
Muli Ben-Yehuda <mulix at mulix.org>
Mikael Berthe <mikael at lilotux.net>
Benoit Boissinot <bboissin at gmail.com>
Brendan Cully <brendan at kublai.com>
Vincent Danjean <vdanjean.ml at free.fr>
Jake Edge <jake at edge2.net>
Michael Fetterman <michael.fetterman at intel.com>
Edouard Gomez <ed.gomez at free.fr>
Eric Hopper <hopper at omnifarious.org>
Alecs King <alecsk at gmail.com>
Volker Kleinfeld <Volker.Kleinfeld at gmx.de>
Vadim Lebedev <vadim at mbdsys.com>
Christopher Li <hg at chrisli.org>
Chris Mason <mason at suse.com>
Colin McMillen <mcmillen at cs.cmu.edu>
Wojciech Milkowski <wmilkowski at interia.pl>
Chad Netzer <chad.netzer at gmail.com>
Bryan O'Sullivan <bos at serpentine.com>
Vicent Seguí Pascual <vseguip at gmail.com>
Sean Perry <shaleh at speakeasy.net>
Nguyen Anh Quynh <aquynh at gmail.com>
Ollivier Robert <roberto at keltia.freenix.fr>
Alexander Schremmer <alex at alexanderweb.de>
Arun Sharma <arun at sharma-home.net>
Josef "Jeff" Sipek <jeffpc at optonline.net>
Kevin Smith <yarcs at qualitycode.com>
TK Soh <teekaysoh at yahoo.com>
Radoslaw Szkodzinski <astralstorm at gorzow.mm.pl>
Samuel Tardieu <sam at rfc1149.net>
K Thananchayan <thananck at yahoo.com>
Andrew Thompson <andrewkt at aktzero.com>
Michael S. Tsirkin <mst at mellanox.co.il>
Rafael Villar Burke <pachi at mmn-arquitectos.com>
Tristan Wibberley <tristan at wibberley.org>
Mark Williamson <mark.williamson at cl.cam.ac.uk>