Error message given when @{u} is used for a branch without its
upstream configured have been clatified.
By Zbigniew Jędrzejewski-Szmek
* zj/upstream-error-message:
i18n: mark @{upstream} error messages for translation
Be more specific if upstream branch is not tracked
Provide better message for barnhc_wiht_tpyo@{u}
Provide branch name in error message when using @{u}
t1507: add tests to document @{upstream} behaviour