Bump version to 0.7.0rc2
feature finish: Choose correct review diff parent

Issue
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feature finish was set up to always generate review diff against
develop. That was, however, not correct in respect to the new Salsita
workflow where a feature branch is merged into release in certain cases
instead. Code review diff generation didn't take this into account.

Solution
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Add one more parameter to BranchReview class constructor that will be
used as --parent in the call to rbt.