Resuming an interrupted build can fail

[Ryan Schmidt]

If a user begins building an older version of a port *after* a newer version of that port has already been committed, and the build is interrupted, then syncing and trying to resume that build later can result in an error. I analyze the problem in more detail in this ticket:

https://trac.macports.org/ticket/29223

Can anybody see a problem with the solution I proposed there?