Fix user filter for users with dashes
Add beginning of implementation for tracking branches

- Create candidate_branches dictionary
- Add find_candidates_branches() method to list branches to merge
Add failing branch merging test to scc merge integration test

- Add push_branch() method to Sandbox test (called by open_pr)
- Refactor test_merge.py to have 2 test classes TestMergePullRequest and
TestMergeBranch
- Add simple TestMergeBranch.testMerge test
Add logic to add tracking branches remotes
Add first implementation of branch merging in merge() method

- Refactor the conflicting logic in local scope function safe_merge()
Fix branch merging logic
Improve formatting of commit message for merged/conflicting branches
Fix flake8 warning
Fix --info message appending
Improve formatting of --info output for candidate branches
Merge remote-tracking branch 'origin/master' into branch_tracking
Merge pull request #153 from sbesson/branch_tracking

Add first implementation of branch tracking in `scc merge`