Don't show gitlink directories when we want "other" files

Don't show gitlink directories when we want "other" files

When "show_other_directories" is set, that implies that we are looking
for untracked files, which obviously means that we should ignore
directories that are marked as gitlinks in the index.

This fixes "git status" in a superproject, that would otherwise always
report that subprojects were "Untracked files:"

Signed-off-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>