Add a way to build from a repo fork

[tombettany]

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Github's technique for PRs is to fork the repo, create a new branch with the new feature and then submit a PR to the original repo. If it is desired to test build the new feature using dr then, currently, the branch must be manually cloned into the original repo and then built. This branch is then redundant and will probably be deleted from the base repo (it will probably also become outdated fairly quickly as well).

This process should be made easier. Maybe something like dr build-fork <repo> <branch>.

dr could potentially do a git remote add <repo>, checkout the branch and then build from there. Would probably need to clean up too ;-)