scalar register: add --no-maintenance option

scalar register: add --no-maintenance option

When registering a repository with Scalar to get the latest opinionated
configuration, the 'scalar register' command will also set up background
maintenance. This is a recommended feature for most user scenarios.

However, this is not always recommended in some scenarios where
background modifications may interfere with foreground activities.
Specifically, setting up a clone for use in automation may require doing
certain maintenance steps in the foreground that could become blocked by
concurrent background maintenance operations.

Allow the user to specify --no-maintenance to 'scalar register'. This
requires updating the method prototype for register_dir(), so use the
default of enabling this value when otherwise specified.

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