read-cache: let verify_path() reject trailing dir separators again

read-cache: let verify_path() reject trailing dir separators again

6e773527b6 (sparse-index: convert from full to sparse, 2021-03-30) made
verify_path() accept trailing directory separators for directories,
which is necessary for sparse directory entries.  This clemency causes
"git stash" to stumble over sub-repositories, though, and there may be
more unintended side-effects.

Avoid them by restoring the old verify_path() behavior and accepting
trailing directory separators only in places that are supposed to handle
sparse directory entries.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>