maintenance: use random minute for scheduling (git-for-windows#597)

* [x] This is an early version of work already under review upstream.

See gitgitgadget#1567 for the version submitted upstream. This is an
early version for the `microsoft/git` fork so we could potentially
include it in a release to our Microsoft partners. Upgrading to a
version with these changes may help with some of the auth problems
plaguing GVFS Cache Servers. (It's not a long-term fix, but would avoid
having fingers pointed in this direction in the future.)

At least on Windows, we should re-run `git maintenance start` as part of
`scalar reconfigure` during the installer. On other platforms, we will
need to rely on users slowly rotating through their local repositories.

> This PR is a recreation of git-for-windows#593 now that the upstream change required
rebasing to resolve header conflicts.