status: don't say 'HEAD detached at HEAD'

status: don't say 'HEAD detached at HEAD'

After using "git checkout --detach", the reflog is left with an entry
like

  checkout: moving from ... to HEAD

This message is parsed to generate the 'HEAD detached at' message in
'git branch' and 'git status', which leads to the not-so-useful message
'HEAD detached at HEAD'.

Instead, when parsing such reflog entry, resolve HEAD to the
corresponding commit in the reflog, so that the message becomes 'HEAD
detached at $sha1'.

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>