When 'git remote prune' was used to delete many refs in a repository
with many refs, a lot of time was spent checking for (now) dangling
symbolic refs pointing to the deleted ref, since warn_dangling_symref()
was once per deleted ref to check all other refs in the repository.

Avoid this using the new warn_dangling_symrefs() function which
makes one pass over all refs and checks for all the deleted refs in
one go, after they have all been deleted.
Signed-off-by: NJens Lindström <jl@opera.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>