Duy Nguyen authored Mar 13, 2013 and Junio C Hamano committed Mar 16, 2013

When a remote ref or a tag is checked out, HEAD is automatically
detached. There is no user-friendly way to find out what ref is
checked out in this case. This patch digs in reflog for this
information and shows "HEAD detached from origin/master" or "HEAD
detached at v1.8.0" instead of "currently not on any branch".

When it cannot figure out the original ref, it shows an abbreviated
SHA-1. "Currently not on any branch" would never display (unless
reflog is pruned to near empty that the last checkout entry is lost).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>