maintenance: add custom config to background jobs

maintenance: add custom config to background jobs

At the moment, some background jobs are getting blocked on credentials
during the 'prefetch' task. This leads to other tasks, such as
incremental repacks, getting blocked. Further, if a user manages to fix
their credentials, then they still need to cancel the background process
before their background maintenance can continue working.

Update the background schedules for our four scheduler integrations to
include these config options via '-c' options:

 * 'credential.interactive=false' will stop Git and some credential
   helpers from prompting in the UI (assuming the '-c' parameters are
   carried through and respected by GCM).

 * 'core.askPass=true' will replace the text fallback for a username
   and password into the 'true' command, which will return a success in
   its exit code, but Git will treat the empty string returned as an
   invalid password and move on.

We can do some testing that the credentials are passed, at least in the
systemd case due to writing the service files.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>