scalar clone: add --no-maintenance option

scalar clone: add --no-maintenance option

When creating a new enlistment via 'scalar clone', the default is to set
up situations that work for most user scenarios. Background maintenance
is one of those highly-recommended options for most users.

However, when using 'scalar clone' to create an enlistment in a
different situation, such as prepping a VM image, it may be valuable to
disable background maintenance so the manual maintenance steps do not
get blocked by concurrent background maintenance activities.

Add a new --no-maintenance option to 'scalar clone'.

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