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.