Show items of interactive git-clean in columns

When there are lots of items to be cleaned, it is hard to see them all
in one screen. Show them in columns instead of in one column will solve
this problem.

Since no longer show items to be cleaned using the "Would remove ..."
format (only plain filenames), we add instructions/warnings as header
before them.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Comments-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Add colors to interactive git-clean

Show help, error messages, and prompt in colors for interactive
git-clean. Re-use config variables from git-add--interactive:

 * color.interactive: When set to always, always use colors for
   interactive prompts and displays. When false (or never),
   never. When set to true or auto, use colors only when the
   output is to the terminal.

 * color.interactive.<slot>: Use customized color for interactive
   git-clean output (like git add --interactive). <slot> may be
   prompt, header, help or error.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Comments-by: Matthieu Moy <Matthieu.Moy@imag.fr>