[issue79] add flag to make hg fail if repo or dirstate locked

[Mirza]

Bryan O'Sullivan wrote:
> New submission from Bryan O'Sullivan <bos at serpentine.com>:
> 
> I mirror some Mercurial repositories at my web site.  When a big snowstorm blew
> through Germany last week, Thomas's web servers died for a few days. 
> Apparently, one of them died in the middle of my cron job doing a pull.  The
> connection never timed out, and the cron job remained stuck.
> 

Similar question: What if computer goes power-off in the middle of 
merging/pulling/anything? Are transactions safe? Will it stay in 
undefined state?

mirza