With a relative path, I can't add tectonic's git repository as a dependency in Cargo.toml; attempting to do so gives:
```
$ cargo build
Updating git repository `https://github.com/tectonic-typesetting/tectonic`
error: failed to load source for a dependency on `tectonic`
Caused by:
Unable to update https://github.com/tectonic-typesetting/tectonic
Caused by:
failed to update submodule `reference_sources`
Caused by:
invalid url `../tectonic-staging.git`: relative URL without a base
```
This lets us track the precise version of the reference sources on which we
are based at any given time. See discussion in the README.
Fortunately, one does not need to check out or build the reference sources to
build Tectonic, which is good because Git submodules are super confusing.