Add support for -i/--interactive to git-clean

Show what would be done and the user must confirm before actually
cleaning. In the confirmation dialog, the user has three choices:

 * Yes: Start to do cleaning.
 * No:  Nothing will be deleted.
 * Edit (default for the first time): Enter edit mode.

When the user chooses the edit mode, the user can input space-
separated patterns (the same syntax as gitignore), and each clean
candidate that matches with one of the patterns will be excluded
from cleaning. When the user feels it's OK, presses ENTER and back
to the confirmation dialog.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Spelling-check-by: Eric Sunshine <sunshine@sunshineco.com>
Comments-by: Matthieu Moy <Matthieu.Moy@imag.fr>