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>

t/t1307-config-blob.sh

@@ -73,4 +73,8 @@ test_expect_success 'can parse blob ending with CR' '
 	test_cmp expect actual
 '
 
+test_expect_success 'config --blob outside of a repository is an error' '
+	test_must_fail nongit git config --blob=foo --list
+'
+
 test_done