If "muddle pull" is given a command line that includes the build description (implicitly or explicitly), it will pull the build description first, and if that pull changed things, will reload the build description and re-evaluate the command line, before pulling the other checkouts requested.
This means that you no longer need to check if "muddle pull" changed the build description and then do it again if it did.
See "muddle help pull" for a more complete description.
If you do want the old behaviour, use "muddle pull -noreload".
As a consequence of this change, the "_just_pulled" value is now set by "muddle checkout" as well as by "muddle pull".
This resolves issue 218.
