Document non-reproducibility arising out of abbreviated git hashes

Many build processes insert abbreviated git hashes into outputs, either through git describe or git rev-parse. This is a source of non-reproducibility as the number of hex characters to include in the abbreviated hash is dependent on the number of objects currently in the local view of the git repo.

Possible solutions:

• git describe --abbrev=12
• git rev-parse --short=12
• Set core.abbrev config