add -i (built-in): use the same indentation as the Perl version

add -i (built-in): use the same indentation as the Perl version

When copying the spaces used to indent non-flat lists in `git add -i`,
one space was appended by mistake. This makes the output of the built-in
version of `git add -i` inconsistent with the Perl version. Let's adjust
the built-in version to produce the same output as the Perl version.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>