Merge branch 'check-protected-branches' into 'master'

Cleaned up/tweaked Project#open_branches

See commit c825404572040a3a45cb9e2b3d3e7d64900f66c9 for the details of the changes and https://gitlab.com/gitlab-org/gitlab-ce/issues/14280#note_4973648 for more information.

See merge request !3985