Specifies the out-variable that will contain the extracted location.
This should be set to `SOURCE_PATH` by convention.
### REPO
The organization or user and repository on GitHub.
### REF
A stable git commit-ish (ideally a tag) that will not change contents. **This should not be a branch.**
For repositories without official releases, this can be set to the full commit id of the current latest master.
If `REF` is specified, `SHA512` must also be specified.
### SHA512
The SHA512 hash that should match the archive (https://bitbucket.com/${REPO}/get/${REF}.tar.gz).
This is most easily determined by first setting it to `1`, then trying to build the port. The error message will contain the full hash, which can be copied back into the portfile.
### HEAD_REF
The unstable git commit-ish (ideally a branch) to pull for `--head` builds.
For most projects, this should be `master`. The chosen branch should be one that is expected to be always buildable on all supported platforms.