status: show tracking branch even no difference

If the current branch has an upstream branch, and there are differences
between the current branch and its upstream, some commands (such as
"git status", "git status -bs", and "git checkout") will report their
relationship. E.g.

    $ git status
    # On branch master
    # Your branch is ahead of 'origin/master' by 1 commit.
    #   (use "git push" to publish your local commits)
    #
    ...

    $ git status -bs
    ## master...origin/master [ahead 1]
    ...

    $ git checkout master
    Already on 'master'
    Your branch is ahead of 'origin/master' by 1 commit.
      (use "git push" to publish your local commits)

But if there is no difference between the current branch and its
upstream, the relationship will not be reported, and it's hard to
tell whether the current branch has a tracking branch or not. And
what's worse, when the 'push.default' config variable is set to
`matching`, it's hard to tell whether the current branch has already
been pushed out or not at all [1].

With this patch, "git status" will report relationship between the
current branch and its upstream counterpart even if there is no
difference.

    $ git status
    # On branch master
    # Your branch is identical to 'origin/master'.
    #
    ...

    $ git status -bs
    ## master...origin/master
    ...

    $ git checkout master
    Already on 'master'
    Your branch is identical to 'origin/master'.

[1]: http://thread.gmane.org/gmane.comp.version-control.git/198703

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