Hallo,
We have the interwiki prefixes '[[:svn:' and '[[:rev:', but last time i checked we don't have any easy way to link to GitWeb or Gerrit.
Maybe it's possible to link to Gerrit using an interwiki prefix, but GitWeb would probably be too complicated and will require a template. (A "link" in the style of '[[File:' with a gazillion parameters is... suboptimal.)
I started a template for doing this: https://translatewiki.net/wiki/Template:Gitweb
It's very naive and early, because i know very little about GitWeb, so feel free to play with it and make it more useful. Also, it's not used much now, so feel free even to break it :)
-- Amir Elisha Aharoni · אָמִיר אֱלִישָׁע אַהֲרוֹנִי http://aharoni.wordpress.com “We're living in pieces, I want to live in peace.” – T. Moore