Add a remote helper to interact with mediawiki (fetch & push)

Implement a gate between git and mediawiki, allowing git users to push
and pull objects from mediawiki just as one would do with a classic git
repository thanks to remote-helpers.

The following packages need to be installed (available on common
repositories):

  libmediawiki-api-perl
  libdatetime-format-iso8601-perl

Use remote helpers in order to be as transparent as possible to the git
user.

Download Mediawiki revisions through the Mediawiki API and then
fast-import into git.

Mediawiki revision number and git commits are linked thanks to notes
bound to commits.

The import part is done on a refs/mediawiki/<remote> branch before
coming to refs/remote/origin/master (Huge thanks to Jonathan Nieder
for his help)

We use UTF-8 everywhere: use encoding 'utf8'; does most of the job, but
we also read the output of Git commands in UTF-8 with the small helper
run_git, and write to the console (STDERR) in UTF-8. This allows a
seamless use of non-ascii characters in page titles, but hasn't been
tested on non-UTF-8 systems. In particular, UTF-8 encoding for filenames
could raise problems if different file systems handle UTF-8 filenames
differently. A uri_escape of mediawiki filenames could be imaginable, and
is still to be discussed further.

Partial cloning is supported using one of:

git clone -c remote.origin.pages='A_Page  Another_Page' mediawiki::http://wikiurl

git clone -c remote.origin.categories='Some_Category' mediawiki::http://wikiurl

git clone -c remote.origin.shallow='True' mediawiki::http://wikiurl

Thanks to notes metadata, it is possible to compare remote and local last
mediawiki revision to warn non-fast forward pushes and "everything
up-to-date" case.

When allowed, push looks for each commit between remotes/origin/master
and HEAD, catches every blob related to these commit and push them in
chronological order. To do so, it uses git rev-list --children HEAD and
travels the tree from remotes/origin/master to HEAD through children. In
other words:

* Shortest path from remotes/origin/master to HEAD
* For each commit encountered, push blobs related to this commit

Signed-off-by: Jérémie Nikaes <jeremie.nikaes@ensimag.imag.fr>
Signed-off-by: Arnaud Lacurie <arnaud.lacurie@ensimag.imag.fr>
Signed-off-by: Claire Fousse <claire.fousse@ensimag.imag.fr>
Signed-off-by: David Amouyal <david.amouyal@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Sylvain Boulmé <sylvain.boulme@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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