fetch: report ref storage DF errors more accurately

fetch: report ref storage DF errors more accurately

When we fail to store a fetched ref, we recommend that the
user try running "git prune" to remove up any old refs that
have been deleted by the remote, which would clear up any DF
conflicts. However, ref storage might fail for other
reasons (e.g., permissions problems) in which case the
advice is useless and misleading.

This patch detects when there is an actual DF situation and
only issues the advice when one is found.

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