OutputPage.php - adding SVN revision number to meta generator tag

I've added <meta name="generator" content="MediaWiki $wgVersion" /> in r36796 (and thanks to Simetrical who tweaked it in r36797).

I think there should be also SVN revision number available there, but I'm not sure if provided patch is the correct way, thus I'm leaving it here to be considered and reviewed.

I've copied two additional functions from Special:Version so now there are two same pieces of code on two different places. So I was wondering it might be better to set up say $wgVersionRevision which can be used on different places then, however somebody more experienced than me would have to do that instead.

Version: 1.13.x
Severity: enhancement

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