add core.usereplacerefs config option

We can already disable replace refs using a command line option or environment variable, but those are awkward to apply universally. Let's add a config option to do the same thing. That raises the question of why one might want to do so universally. The answer is that replace refs violate the immutability of objects. For instance, if you wanted to cache the diff between commit XYZ and its parent, then in theory that never changes; the hash XYZ represents the total state. But replace refs violate that; pushing up a new ref may create a completely new diff. The obvious "if it hurts, don't do it" answer is not to create replace refs if you're doing this kind of caching. But for a site hosting arbitrary repositories, they may want to allow users to share replace refs with each other, but not actually respect them on the site (because the caching is more important than the replace feature). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>