Compare View across forks

Problem to solve

GitLab currently allows users to compare branches within the same repository only. When contributing to upstream open source projects from a fork, it is very useful to compare differences across repositories.

Proposal

The current "compare" function (repository >> compare) only allows to select branch name. We could:

1. Detect when branch/repo is part of a fork network and
2. Show an additionl field for "source repo" (we could use the same pattern we use for new merge requests on forks):

Further info

Here is an example of such a Compare view on github - https://github.com/ellekasai/resumecards/compare/gh-pages...weitzman:master