pull: Clarify "helpful" message for another corner case

pull: Clarify "helpful" message for another corner case

When the remote branch we asked for merging did not exist in the set of
fetched refs, we unconditionally hinted that it was because of lack of
configuration.  It is not necessarily so, and risks sending users for a
wild goose chase.

Make sure to check if that is indeed the case before telling a wild guess
to the user.

Signed-off-by: Junio C Hamano <gitster@pobox.com>