topics: pass quietempty=True to suppress useless output while changing topics
This will help in suppressing the output "0 files updated, 0 file removed, ..."
topics: return early so that other if conditions don't get execute (issue5600)
We were not returning after the _changetopics() call and hence the if's below it
were also executed which resulted in changing of current topic.
This prevent current topic to be updated even when we only affect changeset
unrelated to the working copy.
Not that the rebase output change as it expose a new bug (rebased changeset gain
a topic)