Building the Git version into the code meant that Cargo had to recompile
ptth_relay every time I built. This is annoying and it doesn't add anything.
I changed it to read the Git version from a text file which is absent by
default, and present in the Docker image.
The new method is much nicer and doesn't require the manual make-old-git
step. The top-level command is actually build_and_minimize.bash, which uses
`git archive` to unpack the last Git commit and build with _that_ Dockerfile
and Docker context. This is better for determinism. It's similar to our build
process for that one big project at work.
_