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 <johannes.schindelin@gmx.de>

Author: dscho

t/t3701-add-interactive.sh

@@ -291,6 +291,17 @@ test_expect_success 'different prompts for mode change/deleted' '
 	test_cmp expect actual.filtered
 '
 
+test_expect_success 'correct message when there is nothing to do' '
+	git reset --hard &&
+	git add -p 2>err &&
+	test_i18ngrep "No changes" err &&
+	printf "\\0123" >binary &&
+	git add binary &&
+	printf "\\0abc" >binary &&
+	git add -p 2>err &&
+	test_i18ngrep "Only binary files changed" err
+'
+
 test_expect_success 'setup again' '
 	git reset --hard &&
 	test_chmod +x file &&