builtin/maintenance: mark "--task=" and "--schedule=" as incompatible

builtin/maintenance: mark "--task=" and "--schedule=" as incompatible

The "--task=" option explicitly allows the user to say which maintenance
tasks should be run, whereas "--schedule=" only respects the maintenance
strategy configured for a specific repository. As such, it is not
sensible to accept both options at the same time.

Mark them as incompatible with one another. While at it, also convert
the existing logic that marks "--auto" and "--schedule=" as incompatible
to use `die_for_incompatible_opt2()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>