Use bitmap hash cache on packfiles
2019-04-17: when I wrote this issue, my testbed was misconfigured. In retrospect it's inconclusive to me whether bitmap hash cache helps, or what it helps with. Before we proceed with this we might want to take a closer look at what to expect, and how we can judge if it works.
Experiments in #1382 (closed) suggest that pack.writeBitmapHashCache might speed up the server side part of git clones. There is a downside, however, in that it makes the bitmap files generated on the Gitaly server incompatible with JGit, and because GitLab lets administrators install custom hooks, we cannot rule out that some of our users want to open repositories on Gitaly servers with JGit.
I think we should come back to this after #1382 (closed) has landed and do some more benchmarks to see if we think this is worth it.
Enable the bitmap hash cache and gazette it in the release notes that this may cause a problem if using JGit in a hook