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().
Use communicate() in get_rev_list()

This should allow get_rev_list() to work for submodules
Set no_wait to True in get_rev_list()
Merge pull request #158 from sbesson/checkprs_fix

Fix scc check-prs by using find_branching_point()