Merge branch 'zj/upstream-error-message'

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