status: always show tracking branch even no change

status: always show tracking branch even no change

In order to see what the current branch is tracking, one way is using
"git branch -v -v", but branches other than the current are also
reported. Another way is using "git status", such as:

    $ git status
    # On branch master
    # Your branch is ahead of 'origin/master' by 1 commit.
    ...

But this will not work if there is no change between the current
branch and its upstream. Always report upstream tracking info
even if there is no difference, so that "git status" is consistent
for checking tracking info for current branch. E.g.

    $ git status
    # On branch feature1
    # Your branch is up-to-date with 'github/feature1'.
    ...

    $ git status -bs
    ## feature1...github/feature1
    ...

    $ git checkout feature1
    Already on 'feature1'
    Your branch is up-to-date with 'github/feature1'.
    ...

Also add some test cases in t6040.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>