Patches with `--full-index`?

When using a patch from a gitlab commit, the length of the truncated index can vary based on the size of the repository. I’ve observed that the .diff version contains the full index (as you would obtain by running git diff --full-index, but the .patch version does not.

Is this behavior something we can rely on? I am Homebrew maintainer and we would prefer our external patches to remain stable, i.e., not change their contents over time.