How do I display the git branch in the emacs modeline?

Question

By default the modeline like：

Emacs displays Git-master when I am working in a directory that is under the Git version control system.Now I customize modeline in the file init-modeline.el like this:

(setq-default mode-line-format
    (list
        ;; the buffer name; the file name as a tool tip
        '(:eval (propertize "%b " 'face font-lock-keyword-face
        'help-echo (buffer-file-name)))

        ;; line and column
        "("
        "%02l" "," "%01c"
        ") "
        ......
))

Then add (require 'init-modeline) to init.el.

Now emacs does not display git branch in the emacs modeline, so I add '(vc-mode vc-mode) to init-modeline.el like this:

    (setq-default mode-line-format
    (list
        ;; the buffer name; the file name as a tool tip
        '(:eval (propertize "%b " 'face font-lock-keyword-face
        'help-echo (buffer-file-name)))

        ;; line and column
        "("
        "%02l" "," "%01c"
        ") "

        '(vc-mode vc-mode)
        ......
))

Now modeline shows like this:

It only shows -master.How can I show Git-master?

The value of variable vc-mode in init-mode buffer like:

vc-mode's value is #(" :master" 0 1
  (face sml/vc-edited)
  1 8
  (help-echo "Locally modified file under the Git version control system\nCurrent revision: master\nmouse-1: Version Control menu" face sml/vc-edited local-map
         (keymap
          (mode-line keymap
                     (down-mouse-1 menu-item "Version Control" vc-menu-map :filter vc-menu-map-filter)))
         mouse-face mode-line-highlight))

Local in buffer init.el; global value is nil

  Automatically becomes permanently buffer-local when set.
  This variable may be risky if used as a file-local variable.