feature finish: choose between develop and release

According to Salsita workflow, feature finish should not merge into
develop mindlessly any more, but rather choose to merge into a release
branch if the feature has been already merge into develop at least once.

The release branch is chosen according to the label in Pivotal Tracker,
e.g. if the story is labeled release-1.2.3, feature finish will try to
merge into release/1.2.3 branch.
feature finish: rev 1: Handle labels correctly

There were some issues with handling Pivotal Tracker labels. The former
implementation was discarding existing labels while adding the new one.
This is fixed now.