Show interpreted branch name in error messages

Change the error message when doing: "git branch @{-1}",
"git checkout -b @{-1}", or "git branch -m foo @{-1}"

* was: A branch named '@{-1}' already exists.
* now: A branch named 'bar' already exists.

Signed-off-by: Conrad Irwin <conrad.irwin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>