Add option to UnrebasedPRs to check mismatching PR comments

This commit construct PR dictionaries based on parsed PR body/comments when
listing unrebased PRs. Once the PRs are listed, the from/to rebased comments
are checked against the other branch dictionary and mismatching candidates are
output unless the --no-check is passed to the command.
Refactor static check() method to allow method unit testing

- Move output printing to the notes() method
- Add unit tests for UnrebasedPRs command
Handle multiple sources/targets while checking rebased PR comments

Dictionary returned from static check() method now stores expected values in
a list to support multiple targets/sources.
Fix broken key existence check
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)