path.c: refactor relative_path(), not only strip prefix

Original design of relative_path() is simple, just strip the prefix
(*base) from the absolute path (*abs). In most cases, we need a real
relative path, such as: ../foo, ../../bar. That's why there is another
reimplementation (path_relative()) in quote.c.

Refactor relative_path() in path.c to return real relative path, so
that user can reuse this function without reimplement his/her own.
I will use this method for interactive git-clean later. Some of the
implementations are borrowed from path_relative() in quote.c.

Different results for relative_path() before and after this refactor:

    abs path  base path  relative (original)  relative (refactor)
    ========  =========  ===================  ===================
    /a/b/c/   /a/b       c/                   c/
    /a/b//c/  //a///b/   c/                   c/
    /a/b      /a/b       .                    ./
    /a/b/     /a/b       .                    ./
    /a        /a/b/      /a                   ../
    /         /a/b/      /                    ../../
    /a/c      /a/b/      /a/c                 ../c
    /a/b      (empty)    /a/b                 /a/b
    /a/b      (null)     /a/b                 /a/b
    (empty)   /a/b       (empty)              ./
    (null)    /a/b       (empty)              ./
    (null)    (empty)    (null)               ./

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
test: test relative_path through test-path-utils

Add subcommand "relative_path" in test-path-utils, and add test cases
in t0060.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
quote.c: remove path_relative, use relative_path instead

Since there is an enhanced version of relative_path() in path.c,
remove duplicate counterpart path_relative() in quote.c.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Refactor quote_path_relative, remove unused params

After substitute path_relative() in quote.c with relative_path() from
path.c, parameters (such as len and prefix_len) are obsolete in function
quote_path_relative(). Remove unused parameters and change the order of
parameters for quote_path_relative() function.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Refactor write_name_quoted_relative, remove unused params

After substitute path_relative() in quote.c with relative_path() from
path.c, parameters (such as len and prefix_len) are obsolete in function
write_name_quoted_relative(). Remove unused parameters from
write_name_quoted_relative() and related functions.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: refactor git-clean into two phases

Before introducing interactive git-clean, refactor git-clean operations
into two phases:

 * hold cleaning items in del_list,
 * and remove them in a separate loop at the end.

We will introduce interactive git-clean between the two phases. The
interactive git-clean will show what would be done and must confirm
before do real cleaning.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: add support for -i/--interactive

Show what would be done and the user must confirm before actually
cleaning.

    Would remove ...
    Would remove ...
    Would remove ...

    Remove [y/n]?

Press "y" to start cleaning, and press "n" if you want to abort.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
git-clean: show items of del_list 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 will solve this problem.

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

Show header, help, error messages, and prompt in colors for interactive
git-clean. Re-use config variables, such as "color.interactive" and
"color.interactive.<slot>" for command `git-add--interactive`.

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 will use this framework to add
new actions for interactive git-clean later.

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: add filter by pattern interactive action

Add a new action for interactive git-clean: filter by pattern. When the
user chooses this action, 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 backs to the confirmation dialog.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
git-clean: add select by numbers interactive action

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: add ask each interactive action

Add a new action for interactive git-clean: ask each. It's just like
the "rm -i" command, that the user must confirm one by one for each
file or directory to be cleaned.

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

Add new section "Interactive mode" for documentation of interactive
git-clean.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
test: add t7301 for git-clean--interactive

Add test cases for git-clean--interactive.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
zh_CN.po: translation for git-clean

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