scalar: customize register_dir()'s behavior

scalar: customize register_dir()'s behavior

In advance of adding a --[no-]maintenance option to several 'scalar'
subcommands, extend the register_dir() method to include an option for
how it should handle background maintenance.

It's important that we understand the context of toggle_maintenance()
that will enable _or disable_ maintenance depending on its input value.
Add a doc comment with this information.

Similarly, update register_dir() to either enable maintenance or leave
it alone.

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