rm: fix the new --dir (-d) option to work with -i

rm: fix the new --dir (-d) option to work with -i

* src/remove.c (prompt): Hoist the computation of is_empty, since we'll
need it slightly earlier.
Before, this function would arrange to fail with EISDIR when processing
a directory without --recursive (-r).  Adjust the condition to exempt
an empty directory when --dir has been specified.
Improve comments.
* tests/rm/d-3: New file, to ensure that rm -d -i dir works.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
* THANKS.in: Update.
Reported by Michael Price in http://bugs.gnu.org/12260