init: reset cached config when entering new repo

After we copy the templates into place, we re-read the config in case we copied in a default config file. But since git_config() is backed by a cache these days, it's possible that the call will not actually touch the filesystem at all; we need to tell it that something has changed behind the scenes. Note that we also need to reset the shared_repository config. At first glance, it seems like this should probably just be folded into git_config_clear(). But unfortunately that is not quite right. The shared repository value may come from config, _or_ it may have been set manually. So only the caller who knows whether or not they set it is the one who can clear it (and indeed, if you _do_ put it into git_config_clear(), then many tests fail, as we have to clear the config cache any time we set a new config variable). There are three tests here. The first two actually pass already, though it's largely luck: they just don't happen to actually read any config before we enter the new repo. But the third one does fail without this patch; we look at core.sharedrepository while creating the directory, but need to make sure the value from the template config overrides it. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>