Extend update_dictionary() to merged PRs

This commit should allow non first-parent merged PRs to be checked while
updating the rebase dictionary. It also checks the base of the PR matches the
branch.
merge master -D org -I include -E exclude: PR 92 (Unrebased PRs improvement)
merge master -D org -I include -E exclude: PR 93 (Update submodules command: add first-parent log to the commit message)