built-in add -i: accept open-ended ranges again

The interactive `add` command allows selecting multiple files for some
of its sub-commands, via unique prefixes, indices or index ranges.

When re-implementing `git add -i` in C, we even added a code comment
talking about ranges with a missing end index, such as `2-`, but the
code did not actually accept those, as pointed out in
#2466 (comment).

Let's fix this, and add a test case to verify that this stays fixed
forever.

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

add-interactive.c

@@ -344,7 +344,10 @@ static ssize_t list_and_choose(struct add_i_state *s,
 				if (endp == p + sep)
 					to = from + 1;
 				else if (*endp == '-') {
-					to = strtoul(++endp, &endp, 10);
+					if (isdigit(*(++endp)))
+						to = strtoul(endp, &endp, 10);
+					else
+						to = items->items.nr;
 					/* extra characters after the range? */
 					if (endp != p + sep)
 						from = -1;

t/t3701-add-interactive.sh

@@ -68,6 +68,15 @@ test_expect_success 'revert works (initial)' '
 	! grep . output
 '
 
+test_expect_success 'add untracked (multiple)' '
+	test_when_finished "git reset && rm [1-9]" &&
+	touch $(test_seq 9) &&
+	test_write_lines a "2-5 8-" | git add -i -- [1-9] &&
+	test_write_lines 2 3 4 5 8 9 >expected &&
+	git ls-files [1-9] >output &&
+	test_cmp expected output
+'
+
 test_expect_success 'setup (commit)' '
 	echo baseline >file &&
 	git add file &&