Fix scc check-prs by using find_branching_point()

This commit addresses a tricky issue where a branch originated from an
ancestor commit is merged simultaneously in the source and target branches.
In that case, `git merge-base` will return this ancestor commit instead of
the true branching point. To fix this, CheckPRs now uses find_branching_point
already used in Rebase().

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This PR should fix http://ci.openmicroscopy.org/view/Mgmt/job/SCC-check-prs/.
To test this PR in context, run `python scc/main.py check-prs dev_5_0 develop` from openmicroscopy.git and check the output is valid.