command: Use relative paths in getGitHash

[jmholla]

This seems to be the only part of the code that doesn't expect a relative path to the current working directory, and that's a quirk of git rev-parse with a revision specified. (Without HEAD, this would also work.)

This change ensures we are always looking at files in the current directory so deployment files do not have to be located in the root of a repository.

I have:

• [X] Based work on latest develop branch
• [X] Followed the contribution guide
• [X] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [X] Run the test suite: $(nix-build -A selftest --no-out-link)
• [ ] Updated the changelog
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[jmholla]

This PR is all messed up since I worked off of master.