bug: git-svn seg fault during 'git svn fetch' due to perl < 5.10

In git 1.8.1, when we do 'git svn fetch' on a large repo, we're seeing a
seg fault.  It's caused by git-svn trying to parse a large yaml file
(introduced in 
https://github.com/git/git/commit/68f532f4ba888f277637a94b4a49136054df0540
) which encounters a perl regex stack overflow bug that was present in
perl < 5.10 (https://bugzilla.redhat.com/show_bug.cgi?id=192400).

We'll find a work around, but it'd be nice if there was a config setting
to let us choose not to use the yaml format.

Let me know if there's a better place to report this.

-Durham