When running git blame in a "git show" buffer, we annotate the youngest
version of the file that has the line referenced by the diff line
at cursor.
In case the cursor is on an added or context line, we simply show
the version from the surrounding commit.
When the cursor is on a deleted line, we show the parent commit,
which still has the deleted line. However there is a bug: we use
the line number in the new version of the file. Fix that.
