API: Expose git branch in siteinfo

For browser testing, I need a reliable way to determine the branch in use
on a target wiki.  One could parse the generator name forged when
querying the siteinfo, but that yields:

 MediaWiki 1.24alpha  instead of master
 MediaWiki 1.24wmf2   instead of wmf/1.24wmf2

I am not really willing to reverse engineer the version to figure out
the branch, instead just expose it over the API.

Bug: 62509
Change-Id: If3db8f480ce0d590522741d40fb8e43b753efb41