Describe the bug
hydra truncates git revisions too short, in some places. e.g. src/root/common.tt.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://hydra.nixos.org/build/185573231
2. Take the second hash under Changes: bba87f
3. Run any git command involving this hash against nixpkgs
4. Observe that this hash is not unique

Expected behavior
Produce valid hashes for a given repository.