git: Disable automatic maintenance

While reviewing our distributed tracing, I noticed that git maintenance --auto is being executed in some cases.

By default a number of git commands will run git-maintenance(1) automatically, performing any maintenance tasks that are enabled for that repository. However, Gitaly never enables these tasks and handles maintenance itself, so we gain no benefit from running this process.

There is no harm done to the repository, but we're wasting resources by running it at all. Disable it by setting config option maintenance.auto=0 in our command factory.