git-clean: add toggle flags interactive action

Add new action in the interactive mode, so that the user can change
git-clean flags, such as -x/-X/-d/-ff, and refresh the cleaning
candidates list.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: update document for interactive git-clean

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>