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

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
https://github.com/git-for-windows/git/issues/2466#issuecomment-574142760.

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>