With git you wouldn't have to compromise the stability of MediaWiki's head revision. You could review patches while they are only committed to the patch-writers own local git repository. When you are ready to accept the patch into trunk you do.
A major benefit of this model is that you could have the head revision be defined as the code that is running live on the sites.
git makes it very easy to switch from svn and has lots of svn-compatibility features, including automatically importing your svn repositories, etc.. It has a lot of other benefits as well such as speed (owing in part to the history being on your machine) but the true benefit for MediaWiki is the distributed development model. There is no reason to sacrifice the sanctity of your head just because someone *thinks* their code is ready.
/Brian