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:

 * y/yes:  Start to do cleaning.
 * n/no:   Nothing will be deleted.
 * e/edit: Exclude items from deletion using ignore patterns.

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>
Comments-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Suggested-by: Eric Sunshine <sunshine@sunshineco.com>
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) in interactive mode, we add instructions
and 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 header, help, error messages, and prompt in colors for interactive
git-clean. Re-use config variables for other git commands, such as
git-add--interactive and git-stash:

 * 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>
git-clean: use a git-add-interactive compatible UI

Rewrite menu using a new method `list_and_choose`, which is borrowed
from `git-add--interactive.perl`. We can reused this method later for
more actions.

Please NOTE:

 * Method `list_and_choose` return an array of integers, and
 * it is up to you to free the allocated memory of the array.
 * The array ends with EOF.
 * If user pressed CTRL-D (i.e. EOF), no selection returned.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: interactive cleaning by select numbers

Draw a multiple choice menu using `list_and_choose` to select items
to be deleted by numbers.

User can input:

 *  1,5-7 : select 1,5,6,7 items to be deleted
 *  *     : select all items to be deleted
 *  -*    : unselect all, nothing will be deleted
 *        : (empty) finish selecting, and return back to main menu

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: rm -i style interactive cleaning

Add a "rm -i" style interactive cleaning method. User must confirm one
by one before starting to delete.

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>
git-clean refactor: save some options in clean_flags

Save some options in variable clean_flags, such as -ff (force > 1),
-x (ignored), -X (ignored_only), and -d (remove_directories). We may
change clean_flags later in the interactive git-clean.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean refactor: wrap in scan_clean_candidates

Add new function `scan_clean_candidates`, which determines the del_list
(i.e. the cleaning candidates). This function will be reused later in
the interactive git-clean, so we can change flags of git-clean and
refresh the del_list.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: change clean flags in interactive mode

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>