config: die when --blob is used outside a repository

config: die when --blob is used outside a repository

If you run "config --blob" outside of a repository, then we
eventually try to resolve the blob name and hit a BUG().
Let's catch this earlier and provide a useful message.

Note that we could also catch this much lower in the stack,
in git_config_from_blob_ref(). That might cover other
callsites, too, but it's unclear whether those ones would
actually be bugs or not. So let's leave the low-level
functions to assume the caller knows what it's doing (and
BUG() if it turns out it doesn't).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>