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

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

This implements the `status` command of `git add -i`. The data
structures introduced in this commit will be extended later, as needed.

At this point, we re-implement only part of the `list_and_choose()`
function of the Perl script `git-add--interactive.perl` and call it
`list()`. It does not yet color anything, or do columns, or allow user
input.

Over the course of the next commits, we will introduce a
`list_and_choose()` function that uses `list()` to display the list of
options and let the user choose one or more of the displayed items. This
will be used to implement the main loop of the built-in `git add -i`, at
which point the new `status` command can actually be used.

Signed-off-by: Daniel Ferreira <redacted>
Signed-off-by: Slavica Đukić <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>