Master should have a version number like 0.5.0-dev+572, not +7990,
which is the relevant number if it were 0.4.0-dev.
In the end, I decided to just pull a fresh clone direct from JuliaLang/julia and compile it, and then fiddle with the remotes afterwards.
This means that you have some commits way back in the git history of your fork that have not been merged into JuliaLang/julia's master branch. You probably want to move your commits back up to the top of your branch so they're sitting on top of all the other commits; the way to do this is to run `git pull --rebase origin master` (that will take all the commits that are in your fork and attempt to replay them on top of the current tip of origin's master branch. This is assuming that you want to base your work off of the master branch, and assuming that your "origin" git remote is JuliaLang/julia, if not substitute the appropriate names in wherever you wish)