hg clone --exclude-rev

[Peter Arrenbrecht]

Matt, I played a bit with adding --exclude-rev to clone. We discussed
this as a means to make the recommended strip procedure safer (so you
don't lose other heads you're not aware of). But the semantics seem a
bit unsatisfactory in the general case (all descendants dropped can
lead to dropping merges you really wanted) and it's a lot of changes
if we want to do it properly (need to change wire commands, for
instance).

So I documented the current procedure using `hg clone --rev` with a
note about other heads here:

http://www.selenic.com/mercurial/wiki/PruningDeadBranches

-parren