branch: not report invalid tracking branch

Command "git branch -vv" will report tracking branches, but invalid
tracking branches are also reported. This is because the function
stat_tracking_info() can not distinguish whether the upstream branch
does not exist, or nothing is changed between one branch and its
upstream.

This patch changes the return value of function stat_tracking_info().
Only returns false when there is no tracking branch or the tracking
branch is invalid, otherwise true. If the caller does not like to
report tracking info when nothing changed between the branch and its
upstream, simply checks if num_theirs and num_ours are both 0.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
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 may need to do
something. 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>