This reverts af43dd6a with the aim of
reducing Gitaly server resouce usage when fetching Git data for CI
builds. Instead of fetching the pipeline ref by SHA, we go back to
fetching it by name 'refs/pipelines/1234'. This causes `git fetch` to
use the 'ref-prefix' option [1] when fetching refs. At the time of
af43dd6a, the server side
implementation of 'ref-prefix' was very inefficient. This got fixed in
Git by [2].

Currently each CI fetch causes a full ref walk on the Gitaly server.
By going back to fetching by ref, this changes to a filtered refwalk
which is much faster. Especially on repositories with many (server
side) refs, such as gitlab-org/gitlab.

[1]: https://gitlab.com/gitlab-org/gitlab-git/-/blob/v2.33.0/Documentation/technical/protocol-v2.txt#L191-193
[2]: https://gitlab.com/gitlab-org/gitlab-git/-/commit/b3970c702cb0acc0551d88a5f34ad4ad2e2a6d39