This reimplements the ancient "-q" option to "git diff-files" that
was inherited from "show-diff -q" in terms of "--diff-filter=d". We
will be deprecating the "-q" option, so let's issue a warning when
we do so.
Incidentally this also tentatively fixes "git diff --no-index" to
honor "-q" and hide deletions; the use will get the same warning.
We should remove the support for "-q" in a future version but it is
not that urgent.