Commit hashes should be reproducible.

What I did: Ran `tsb prebuild` against the same `patches.yml` twice

Expected result: Same commit hashes each time

Actual result: Different commit hashes each time

More info:

Given a `repos.yml` and `patches.yml` file, `tsb prebuild` should produce the same commit hashes each time for the cherry-picked commits. See other projects like , which go to great lengths to produce the same commit hashes each time.

Start by running `tsb prebuild` twice with the same `repos.yml` and `patches.yml`, then compare the hashes of the cherry-picked commits. Inspect the prebuild commit-creation path for inputs that vary between runs. Done means identical inputs consistently produce identical commit hashes.

Written by the indexing model from the issue text.