Git: add branch config for YouCompleteMe submodule
Git: add branch config for YouCompleteMe submodule

So, the PR isn't going to be merged (as per discussion), so I am going
to have to track my fork permanently.

This commit sets up `.gitmodules` so that when we update the bundle, we
stay pointed at the right branch of my fork.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent