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