Revert breaking upstream changes for Gollum 5.x
Background
We're currently upgrading to Rugged version 1.0: gitaly#2856 (closed)
In !6 (merged) we pulled in all upstream changes from the gollum-rugged-adapter gem, because some of them fix known bugs and change some behaviour for Rugged 1.x.
In https://gitlab.com/gitlab-org/gollum-lib/-/merge_requests/18 we wanted to pull in the new version of our forked gem, and noticed that some of the changes were made for Gollum 5.x, which we don't plan to upgrade to in the near future (more details on that in https://gitlab.com/gitlab-org/gollum-lib/-/merge_requests/18#note_406899518).
What does this MR do?
I went through the merged upstream commits and saw that most of the changes are useful/desired for us, so in the end I only reverted those that seemed related to Gollum API changes and caused our specs to fail. That functionality was mostly around diffing/reverting/searching, which we don't expose in Gitaly/GitLab so it should be fairly safe.
We're still using the external specs from the adapter_specs gem, although I had to revert them to an earlier commit (the gem is unreleased and only available through GitHub, unfortunately no tags either).
Test coverage
The changes in this MR are additionally verified through the pipelines in these MRs: