One of the instructions for setting up devstack is to run make dev.clone, which clones a number of git repos into the parent directory. For the past two years, these clones have been created with --single-branch, so that running git fetch only fetched a single branch. This made development difficult and was confusing to a number of people.
As of https://github.com/edx/devstack/pull/628 this is no longer the case, and the clones will see all remote branches. (This does not affect shallow clones.)
If you’d like to fix existing clones, you can identify the odd ones by the output of git config --local remote.origin.fetch. Normally this has the value of +refs/heads/*:refs/remotes/origin/* but you might see +refs/heads/master:refs/remotes/origin/master if you cloned within the past two years. The same git config command can be used to correct this if you wish.