built-in add -i: implement the `help` command

built-in add -i: implement the `help` command

This imitates the code to show the help text from the Perl script
`git-add--interactive.perl` in the built-in version.

To make sure that it renders exactly like the Perl version of `git add
-i`, we also add a test case for that to `t3701-add-interactive.sh`.

Signed-off-by: Slavica Đukić <slawica92@hotmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>