built-in add -i: implement the main loop

built-in add -i: implement the main loop

The reason why we did not start with the main loop to begin with is that
it is the first user of `list_and_choose()`, which uses the `list()`
function that we conveniently introduced for use by the `status`
command.

In contrast to the Perl version, in the built-in interactive `add`, we
will keep the `list()` function (which only displays items) and the
`list_and_choose()` function (which uses `list()` to display the items,
and only takes care of the "and choose" part) separate.

The `list_and_choose()` function, as implemented in
`git-add--interactive.perl` knows a few more tricks than the function we
introduce in this patch:

- There is a flag to let the user select multiple items.

- In multi-select mode, the list of items is prefixed with a marker
  indicating what items have been selected.

- Initially, for each item a unique prefix is determined (if there
  exists any within the given parameters), and shown in the list, and
  accepted as a shortcut for the selection.

These features will be implemented in the C version later.

This patch does not add any new main loop command, of course, the
built-in `git add -i` still only supports the `status` command. The
remaining commands to follow over the course of the next commits.

To accommodate for listing the commands in columns, preparing for the
commands that will be implemented over the course of the next
patches/patch series, we teach the `list()` function to do precisely
that.

Note that we only have a prompt ending in a single ">" at this stage;
later commits will add commands that display a double ">>" to indicate
that the user is in a different loop than the main one.

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