emacs: use vc-git-grep instead of helm-git-grep

Think I prefer the normal grep/compilation UI to helm's. Or at the very
least it's more standard so I should get used to it. It's also what
ggtags uses so that's more consistent.

magit-grep is just a wrapper round regular grep at the moment. The
maintainer will implement it properly at some point but not for quite a
while. So just use vc-git-grep in the meantime.