git-clean: update document for interactive git-clean

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

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.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: add wrapper scan_clean_candidates

Add new wrapper `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: 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>
zh_CN.po: translation for git-clean

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