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().
merge master -D org: PR 157 (Milestone command)
merge master -D org: PR 158 (Fix scc check-prs by using find_branching_point())
merge master -D org

Repository: openmicroscopy/snoopycrimecop
Already up-to-date.

Merged PRs:
  # PR 157 sbesson 'Milestone command'
  # PR 158 sbesson 'Fix scc check-prs by using find_branching_point()'

Generated by SCC-merge#289 (http://ci.openmicroscopy.org/job/SCC-merge/289/)