Use regular expressions to retrieve pull request numbers

- Use re.search when parsing git log
- Use re.match when parsing --rebased comments
- Add test with trailing period to TestUnrebasedPRs
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)