status: always show tracking branch even no change

If the current branch has an upstream branch, and there are changes
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.
    ...

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

    $ git checkout master
    Already on 'master'
    Your branch is ahead of 'origin/master' by 1 commit.
    ...

With this patch, the relationship between the current branch its
upstream will be reported always even if there is no difference
between them. E.g.

    $ 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'.
    ...

Then if there is no tracking info reported, the user will know that
maybe the current branch is a new branch that needs to be pushed
out, or maybe it's a branch which should add remote tracking settings.

This patch changes the return value of function stat_tracking_info()
from 0 (not report) to 1 (report), when the current branch and its
remote tracking branch point to the same commit. Also add some test
cases in t6040.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
translation for remote.c

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