Add UI to indicate position relative to default/upstream branch
### Description
It would be useful to know the position of branches relative to the default or upstream branch so that you could, for example, update a branch so that it does not get too far behind. In a self-contained repository (not a fork), something that would be great to see on the "homepage" of a branch is:
~~~
This branch is 1 commit ahead, 2 commits behind `master`
~~~
Or if you are on a fork:
~~~
This branch is 1 commit ahead, 2 commits behind `GitLab.org/master`
~~~
----
This is currently displayed for the branches index:

The proposal is to also display this information on the individual tree view for the branch. The tree view is displayed when selecting a new branch in the dropdown of the project view and when choosing to view an individual branch from the branches index.
### Proposal
If I'm not mistaken, Git can easily tell you which commits your branch has or does not have relative to the `master` or `upstream` branch. Thus, it would be a matter of surfacing this information somewhere, perhaps over the latest commit information as it is on GitHub.
### Links / references
If you have a GitHub account, you can corroborate what I described above.
### Documentation blurb
This is not really an interactive feature but rather information that is just surfaced on the UI.
issue