status/commit: 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 commit command uses the wt_status* functions to produce status
output for human consumption (when forking an editor or with --dry-run),
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 which still ignores submodules according to their configuration.
This will be fixed in a follow-up commit.

Change t7508 to reflect this new behavior and add three new tests to show
that a single staged submodule configured to be ignored will be committed
when the status output is generated and won't be if not. Also update the
documentation of the ignore config options accordingly.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
commit -m: commit staged submodules regardless of ignore config

The previous commit fixed the problem that the staged but that ignored
submodules did not show up in the status output of the commit command and
weren't committed afterwards either. But when commit doesn't generate the
status output (e.g. when used in a script with '-m') the ignored submodule
will still not be committed. This is because in that case a different code
path is taken which calls index_differs_from() instead of calling the
wt_status functions.

Fix that by calling index_differs_from() from builtin/commit.c with a
diff_options argument value that tells it not ignore any submodule changes
unless the '--ignore-submodules' option is used. Even though this option
isn't yet implemented for cmd_commit() but only for cmd_status() this
prepares cmd_commit() to correctly handle the '--ignore-submodules' option
later. As status and commit share the same ignore_submodule_arg variable
this makes the code more robust against accidental breakage and documents
how to correctly call index_differs_from().

Change the expected result of the test documenting this problem from
failure to success.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>