Support tracking branches

Problem to solve

To support git workflows with multiple active branches, it would be nice to support their relationship within gitlab. E.g. there might be a branch relationship like

• master
  • next
    • feature-foo
    • feature-bar
• v00/master
  • v00/next
• v01/master
  • v01/next

This relationship should be visible within gitlab itself; e.g.

• the "xxx commits behind/yyy commits ahead" statistics in repository/branches should be relative to the parent/tracking branch; etm, they are based on "default" branch

• merge requests should be relative to the tracking branch; atm. there must be done multiple steps to change the base branch

Intended users

• Developers
• Project Maintainers