Call for testing: port migrate

[Joshua Root]

A 'migrate' action has been added to MacPorts base in git on the master 
branch. If you are comfortable with checking out the git repo, 
installing MacPorts from source, running potentially buggy pre-release 
code, and reporting any issues, we would greatly appreciate you giving 
it a try.

The new action automates all parts of the existing Migration procedure 
apart from updating Xcode and the CLTs. After updating your OS to a new 
major version or transferring to a new machine with a different CPU 
architecture, you should be able to simply run:

sudo port migrate

and MacPorts base will first be rebuilt, and then all ports that need to 
be reinstalled to be compatible with your current system will be. Build 
failures should be handled as gracefully as possible and reported at the 
end.

Thanks to Umesh Singla for doing the initial work on this feature, and 
to Clemens Lang for doing much of the work of completing and updating 
the code.

- Josh