add-interactive: fix deletion of non-empty files

add-interactive: fix deletion of non-empty files

Commit 24ab81a fixed the deletion of empty files, but broke
deletion of non-empty files. The approach it took was to
factor out the "deleted" line from the patch header into its
own hunk, the same way we do for mode changes. However,
unlike mode changes, we only showed the special "delete this
file" hunk if there were no other hunks. Otherwise, the user
would annoyingly be presented with _two_ hunks: one for
deleting the file and one for deleting the content.

This meant that in the non-empty case, we forgot about the
deleted line entirely, and we submitted a bogus patch to
git-apply (with "/dev/null" as the destination file, but not
marked as a deletion).

Instead, this patch combines the file deletion hunk and the
content deletion hunk (if there is one) into a single
deletion hunk which is either staged or not.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>