Hg update should prefer heads without bookmarks

[Matt Mackall]

On Fri, 2015-05-29 at 13:09 -0700, Nathan Goldbaum wrote:
> The last commit was in 2012, and the last substantial changes were in 2011:
> 
> https://bitbucket.org/bos/hgbook/commits/all

And I don't think those are even live on the website. As far as I can
tell, the last changeset deployed was 3a8e7 in 2009.

Quite a number of things have changed since then. As I mentioned, the
book is current as of Mercurial 1.2. So while it's still an ok
introduction to the basic concepts, it's by no means authoritative on
much of anything anymore.

-- 
Mathematics is the supreme nostalgia of our time.