t3701: verify the shown messages when nothing can be added

t3701: verify the shown messages when nothing can be added

In preparation for re-implementing `git add -p` in pure C (where we will
purposefully keep the implementation of `git add -p` separate from the
implementation of `git add -i`), let's verify that the user is told the
same things as in the Perl version when the diff file is either empty or
contains only entries about binary files.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>