builtin/pack-objects.c: avoid iterating all refs

builtin/pack-objects.c: avoid iterating all refs

In git-pack-objects, we iterate over all the tags if the --include-tag
option is passed on the command line. For some reason this uses
for_each_ref which is expensive if the repo has many refs. We should
use for_each_tag_ref instead.

Because the add_ref_tag callback will now only visit tags we
simplified it a bit.

The motivation for this change is that we observed performance issues
with a repository on gitlab.com that has 500,000 refs but only 2,000
tags. The fetch traffic on that repo is dominated by CI, and when we
changed CI to fetch with 'git fetch --no-tags' we saw a dramatic
change in the CPU profile of git-pack-objects. This lead us to this
particular ref walk. More details in:
https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/746#note_483546598

Signed-off-by: Jacob Vosmaer <jacob@gitlab.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>