Merge 'add-p-many-files'

This topic branch allows `add -p` and `add -i` with a large number of
files. It is kind of a hack that was never really meant to be
upstreamed. Let's see if we can do better in the built-in `add -p`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t3701-add-interactive.sh

@@ -1166,6 +1166,27 @@ test_expect_success 'checkout -p patch editing of added file' '
 	)
 '
 
+test_expect_success EXPENSIVE 'add -i with a lot of files' '
+	git reset --hard &&
+	x160=0123456789012345678901234567890123456789 &&
+	x160=$x160$x160$x160$x160 &&
+	y= &&
+	i=0 &&
+	while test $i -le 200
+	do
+		name=$(printf "%s%03d" $x160 $i) &&
+		echo $name >$name &&
+		git add -N $name &&
+		y="${y}y$LF" &&
+		i=$(($i+1)) ||
+		exit 1
+	done &&
+	echo "$y" | git add -p -- . &&
+	git diff --cached >staged &&
+	test_line_count = 1407 staged &&
+	git reset --hard
+'
+
 test_expect_success 'show help from add--helper' '
 	git reset --hard &&
 	cat >expect <<-EOF &&