Merge branch 'ds/background-maintenance-with-credential'

Merge branch 'ds/background-maintenance-with-credential'

Background tasks "git maintenance" runs may need to use credential
information when going over the network, but a credential helper
may work only in an interactive environment, and end up blocking a
scheduled task waiting for UI.  Credential helpers can now behave
differently when they are not running interactively.

* ds/background-maintenance-with-credential:
  scalar: configure maintenance during 'reconfigure'
  maintenance: add custom config to background jobs
  credential: add new interactive config option