Install mw-to-git into production location. (Closes: #718395)

Install mw-to-git into production location. (Closes: #718395)

This makes the following things work (tested):

 $ git clone -c remote.origin.pages='PluginDemo' \
   mediawiki::https://evolvis.org/plugins/mediawiki/wiki/evolvis

(Cloning from an Evolvis (FusionForge) Wiki; pushing not tested
because the Wiki in question does not allow anonymous write access.)

 $ git clone -c remote.origin.pages='User:mirabilos' \
   mediawiki::http://en.wikipedia.org/w user_mirabilos
 $ cd user_mirabilos; jupp User:Mirabilos.mw
 $ git commit -a -m …; git push

(Cloning from Wikipedia, editing, pushing; shows up as anonymous
commit from my IPv4 address.)

This functionality requires two additional packages installed;
adjust Suggests appropriately.

“git help mw”, “git help remote-mediawiki” do not work; but then,
upstream does not provide a lot of help in the first place. If
it’s necessary I may consider writing appropriate manpages after
I’ve used this for a while and learned of its possibilities.

[jn: cleaned up build rules; moved to separate git-mediawiki package]

Signed-off-by: Thorsten Glaser <t.glaser@tarent.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>