Reduce time spent in Git pushes
Even when using an empty repository on CephFS quite a bit of time is spent in processing push payloads (a handful of seconds). Quickly looking at the code used for checking permissions it seems we're doing a lot of work inside loops (when this may not be needed). Let's aim to get 95% of all pushes to complete in 1 second. We'll also need to make sure as much as possible is instrumented. It's also possible a lot of time is spent in gitlab-shell, which is currently pretty much un-instrumented.