builtin/maintenance: improve readability of strategies

builtin/maintenance: improve readability of strategies

Our maintenance strategies are essentially a large array of structures,
where each of the tasks can be enabled and scheduled individually. With
the current layout though all the configuration sits on the same nesting
layer, which makes it a bit hard to discern which initialized fields
belong to what task.

Improve readability of the individual tasks by using nested designated
initializers instead.

Suggested-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>