Return the most-recently-updated folder starting with a common prefix.

This is useful where your project branches all start with the same name as the project, e.g.
$RAILS_APPS/wizzo -- trunk
$RAILS_APPS/wizzo_feature1 -- branch for feature1
$RAILS_APPS/wizzo_feature2 -- branch for feature2

$ latest_branch --app=wizzo
--> returns absolute folder path for most recently modified folder