Add git HEAD date to Special:Version for core and extensions

Add git HEAD date to Special:Version for core and extensions

The patch adds the localised commit date of i) core and ii) extensions
in the Special:Version page tables. It requires the Git version control
system being installed, which is checked during the installation.

Introduces a new parameter for the git binary in DefaultSettings.php:
$wgGitBin = '/usr/bin/git';

Patch authored by DaSch <dasch@daschmedia.de> and updated and fixed
by Wikinaut<mail@tgries.de>.

Bug: 38783
Change-Id: I0931400ecacf91ed2ab4fc7aa46dceac17661768