remote rename/remove: handle branch.<name>.pushRemote config values

remote rename/remove: handle branch.<name>.pushRemote config values

When renaming or removing a remote with

    git remote rename X Y
    git remote remove X

Git already renames/removes any config values from

    branch.<name>.remote = X

to

    branch.<name>.remote = Y

As branch.<name>.pushRemote also names a remote, it now also renames
or removes these config values from

    branch.<name>.pushRemote = X

to

    branch.<name>.pushRemote = Y

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>