Add GitInfo class.

Add GitInfo class.

This adds GitInfo which is used by Special:Version to get the SHA1 of
the git repo.
It's also useful for development to put blocks of config in
LocalSettings.php that only apply to branches where you are developing
large features:

  if ( GitInfo::currentBranch() == 'myrewriteproject' ) {
    // […]
  }

Change-Id: I2a76662bb40080be6556d4edf60ac6714f36a727