I9eabf0b352 introduced a new field in CherryPickInput: topic. This
field allows entering a topic which will be added for the newly
created change.
However, it is possible to cherry-pick a change such that the target
already exists, and then a new patchset would be created rather than
a new change. In this case, currently, the topic doesn't change.
This change fixes this bug: now, a topic will be added even when
creating a patchset (and not only a change).
Change-Id: I37517f15529039337146417259bb672b14fadfe5
1f83c200a0