[BUG] org-agenda-skip-*-if-* variables change log mode

[Samuel Wales <samologist@gmail.com>]

at least i /think/ this is a bug.

i like to set log mode to show everything.  i also go to
previous dates to find out what i did.

however, the skip variables can change log mode.

===

consider org-agenda-skip-scheduled-if-done.  showing is
useful for "what was doneified at an unspecified date but
was first scheduled today or on that day".

it is not intended for "what you did today or on that day".

in my case, the latter is useful, but the former is not.

===

if you have it set to the default of nil, and have log mode
on, it will redundantly show a closed non-timed scheduled in
both the timed section as a closed log mode item, and in the
non-timed section.

thus it is tempting to set it to t to remove the redundancy.

===

but if you do that, YOU ARE ALSO REMOVING ITEMS FROM LOG
MODE.  (note: untested in -Q.)

assuming log mode is set to show closed tasks:

- if it was UNSCHEDULED and done, it will show in log mode.
- if it was SCHEDULED and done, it will not show in log mode.

this absence might get lost.  you might notice closed tasks,
and not notice that you turned off scheduled tasks.

===

i doubt that many want to skip scheduled done tasks in log
mode, especially when unscheduled done tasks show.

imagine a user with a "memacs type mindset".  he or she
would probably be surprised that this variable omits items
from log mode.

it seems impossible to skip without also affecting log mode.

my suggested fix is a new or changed non-nil value to skip without
changing log mode.

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