add -i (built-in): prevent the `reset` "color" from being configured

add -i (built-in): prevent the `reset` "color" from being configured

The Perl version of that command sneakily uses `git config --get-color`
to figure out the ANSI sequence to reset the color, but passes the empty
string and therefore cannot actually match any config entry.

This was missed when re-implementing the command as a built-in command.
Let's fix this, preventing the `reset` sequence from being overridden
via the config.

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