Revert "Add simple check for mergeable status in find_candidates()"

This reverts commit 920bb3718a2818bbf9ef86414d45ff343dd94076.
Merge pull request #155 from sbesson/non_mergeable_revert

Revert "Add simple check for mergeable status in find_candidates()"
Add comment for PullRequests with a description

When passing `--comment` to scc merge, Pull Request with no description
should be commented during the merge operation.
Flake8 the entire repository including setup.py
Update PR before fetching issue comments
Retrieve comments using issue and test comments number
Add new Milestone command to scc

- use argparse sub_parsers to define `scc milestone COMMAND`
- add a flag to Command.__init__ to prevent setting the default command
- add first implementation of `scc milestone list`
Add milestone creation implementation
Add permissions check for scc milestone create
Add missing blank line
Merge pull request #156 from sbesson/description_comment

Add comment for PullRequests without a description
Fix scc check-prs by using find_branching_point()

This commit addresses a tricky issue where a branch originated from an
ancestor commit is merged simultaneously in the source and target branches.
In that case, `git merge-base` will return this ancestor commit instead of
the true branching point. To fix this, CheckPRs now uses find_branching_point
already used in Rebase().
Wrap Milestone in an object to display extended information

Use Milestone.__str__ in scc milestone list
Use communicate() in get_rev_list()

This should allow get_rev_list() to work for submodules
Add milestone delete command and use command logger
Move get_milestones() and get_milestone() to GitHubRepository class
Fix description setting in `scc milestone create`
Add close subcommand to scc milestone
Set no_wait to True in get_rev_list()
Merge pull request #158 from sbesson/checkprs_fix

Fix scc check-prs by using find_branching_point()