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

After `status` and `help`, it is now time to port the `update` command
to C, the second command that is shown in the main loop menu of `git add
-i`.

This `git add -i` command is the first one which lets the user choose a
subset of a list of files, and as such, this patch lays the groundwork
for the other commands of that category:

- It teaches the `print_file_item()` function to show a unique prefix
  if we found any (the code to find it had been added already in the
  previous patch where we colored the unique prefixes of the main loop
  commands, but that patch uses the `print_command_item()` function to
  display the menu items).

- This patch also adds the help text that is shown when the user input
  to select items from the shown list could not be parsed.

- As `get_modified_files()` clears the list of files, it now has to take
  care of clearing the _full_ `prefix_item_list` lest the `sorted` and
  `selected` fields go stale and inconsistent.

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

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