Initial refactoring of UnrebasedPRs command

- Add branch check for rebased PRs (to support more than 2 branches)
- Save all rebased links in the self.links attribute
- Simplify check_links() method
Fix unrebased PRs detection condition
Restore check_directed_links() and read_links() as static methods

- Fix test_unrebased_prs unit tests
Define link type for all parsing output results
Refactor test_unrebasedprs unit test to use pytest parametrize fixture
Add comment lines
Implement caching for PullRequest get_issue() and get_comments()
Move check for rebased PRs to a method and use merged attribute

This commit should fix the failing loop break and speed up performance as we
use the PullRequest attribute instead of making API calls for the merge state
Test unrebased_prs integration test to compare dev_4_4 and develop
Merge pull request #133 from sbesson/unrebased_prs_multibranch

Refactor unrebased prs command