Set changes in return dict

This resolves a case in which an installed package is rebuilt but the
version does not change. Since the package db doesn't appear to have
changed, the ports.install function returns an empty dict, leading the
ports.installed state to believe there are no changes, thus setting the
color of the highstate output to green instead of cyan.

This changes the behavior of the ports.install function so that it sets
the return dict when the make install command succeeds, in cases where
no changes to the pkgdb are detected.

Fixes #9962.