remote: repack packed-refs once when deleting multiple refs

When 'git remote rm' or 'git remote prune' were used in a repository
with many refs, and needed to delete many remote-tracking refs, a lot
of time was spent deleting those refs since for each deleted ref,
repack_without_refs() was called to rewrite packed-refs without just
that deleted ref.

To avoid this, call repack_without_refs() first to repack without all
the refs that will be deleted, before calling delete_ref() to delete
each one completely.  The call to repack_without_ref() in delete_ref()
then becomes a no-op, since packed-refs already won't contain any of
the deleted refs.

Signed-off-by: Jens LindstrÃ¶m <jl@opera.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
remote prune: optimize "dangling symref" check/warning

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: Jens LindstrÃ¶m <jl@opera.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>