This code was put in place in preparation for a feature that never
shipped. Namely, we wanted to use the commit hash for the base branch so
we can get an accurate `git log` involving the changes in a pull
request. However, getting the commit hash from API is not the way to go
because the latest commit might not be available in the person's local
repository, and using a local tracking branch for base such as
`origin/master` works quite well in most cases without dereferencing it.
219e3ecc5b