status/commit: always show staged submodules regardless of ignore config

Currently setting submodule.<name>.ignore and/or diff.ignoreSubmodules to
"all" suppresses all output of submodule changes for the diff family,
status and commit. For status and commit this is really confusing, as it
even when the user chooses to record a new commit for an ignored submodule
by adding it manually this change won't show up under the to-be-committed
changes. To add insult to injury, a later "git commit" will error out with
"nothing to commit" when only ignored submodules are staged.

Fix that by making wt_status always print staged submodule changes, no
matter what ignore settings are configured. The only exception is when the
user explicitly uses the "--ignore-submodules=all" command line option, in
that case the submodule output is still suppressed. This also makes "git
commit" work again when only modifications of ignored submodules are
staged, as that command uses the "commitable" member of the wt_status
struct to determine if staged changes are present. But this only happens
when the status is shown in an editor and the status output is shown
there, in all other cases (e.g. when run in a script with '-m') another
code path is taken which uses index_differs_from() to determine if any
changes are staged.

Change t7508 to reflect this new behavior and add a new test to show that
a single staged submodule configured to be ignored will be committed. Also
mention this in the documentation of the ignore config options.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! submodule summary --for-status: show staged submodules regardless of ignore config

TODO: print unstaged submodules in summary output for --ignore-submodules=none
!!! WIP !!! gitk: show staged submodules regardless of ignore config

TODO: git gui too please