bisect: Store first bad commit as comment in log file

bisect: Store first bad commit as comment in log file

When bisect successfully finds a single revision, the first bad commit
should be shown to human readers of 'git bisect log'.

This resolves the apparent disconnect between the bisection result and
the log when a bug reporter says "I know that the first bad commit is
$rev, as you can see from $(git bisect log)".

Signed-off-by: Torstein Hegge <hegge@resisty.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>