status: reinstate --show-ignored-directory as a deprecated option

It was a bad idea to just remove that option from Git for Windows
v2.15.0, as early users of that (still experimental) option would have
been puzzled what they are supposed to do now.

So let's reintroduce the flag, but make sure to show the user good
advice how to fix this going forward.

We'll remove this option in a more orderly fashion when we're certain
that the option is no longer used (previous Visual Studio versions
relied on it).

The option is deprecated now, therefore we make sure that keeps saying
so until we finally remove it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>