fetch: give a hint to the user when local refs fail to update

fetch: give a hint to the user when local refs fail to update

There are basically two categories of update failures for
local refs:

  1. problems outside of git, like disk full, bad
     permissions, etc.

  2. D/F conflicts on tracking branch ref names

In either case, there should already have been an error
message. In case '1', hopefully enough information has
already been given that the user can fix it. In the case of
'2', we can hint that the user can clean up their tracking
branch area by using 'git remote prune'.

Note that we don't actually know _which_ case we have, so
the user will receive the hint in case 1, as well. In this
case the suggestion won't do any good, but hopefully the
user is smart enough to figure out that it's just a hint.

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