git-add--interactive: allow list (un)selection by regular expression

Teach `list_and_choose` to allow `/regexp` and `-/regexp` syntax to
select items based on regular expression match.

This feature works in all list menus in `git-add--interactive`, and is not
limited to file menus only.

For instance, in file lists, `/\.c$` will select all files whose extension
is `.c`.  In option menus, such as the main menu, `/pa` could be used to
choose the `patch` option.

Signed-off-by: Aarni Koskela <akx@iki.fi>