`prodcommit` uses `git describe` to make a human-friendly name for before and
after revisions of submodules. Usually this simply resolves to a corresponding
tag name, however occasionally the revision is not tagged, and `git describe`
makes up a name that includes the short commit hash of the revision, as well
as the tag of the nearest ancestor of the revision that has one.
However, it turns out that length of any short commit hash generated by
`git describe` is (a) subject to a per-user override using `git config`, and
(b) defaults to 7 characters, which contrasts to the 8 used by GitLab.
To allow for easy cross-referencing against GitLab, change `prodcomit` so that
it forces `git describe` to use 8-character short commit hashes.