Re: psql tabcomplete - minor bugfix - tabcomplete for SET ROLE TO xxx

The following review has been posted through the commitfest application:
make installcheck-world:  not tested
Implements feature:       tested, passed
Spec compliant:           tested, passed
Documentation:            not tested

I agree with Peter that "We don't tab-complete everything we possibly could", but using tabs after "SET ROLE TO "
provides"DEFAULT" as an option which seems wrong.
 
This patch adds list of roles over there, which I guess good to have than giving something unusual.

I reviewed this straight forward patch and looks good to me.

Since we might not want this, review is done and thus passing it to committer to decide.

The new status of this patch is: Ready for Committer