Merge branch 'rr/die-on-missing-upstream'

When a reflog notation is used for implicit "current branch", we
did not say which branch and worse said "branch ''".

* rr/die-on-missing-upstream:
sha1_name: fix error message for @{<N>}, @{<date>}
sha1_name: fix error message for @{u}

diff --cc sha1_name.c
Simple merge