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 <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3203-branch-output.sh

@@ -106,7 +106,7 @@ EOF
 	test_i18ncmp expect actual
 '
 
-test_expect_failure 'git branch shows detached HEAD properly after checkout --detach' '
+test_expect_success 'git branch shows detached HEAD properly after checkout --detach' '
 	git checkout master &&
 	cat >expect <<EOF &&
 * (HEAD detached at $(git rev-parse --short HEAD^0))