Using `git rev-parse --short` will adapt the number of characters based on the number of commits in the repository. In a development checkout of Discourse, it'll currently output 10 characters, making the chance of a collision minuscule.
However, in production we use a shallow clone of the Discourse repository. That means that `git rev-parse --short` only outputs 8 characters. On the `main` branch (with almost 50k commits), we currently have 6 7-character collisions, and no 8-character collisions. However, when looking beyond the `main` branch, 8-character collisions [have been observed](https://meta.discourse.org/t/193320/3).
This commit updates docker_manager to use the full commit hash internally, and for GitHub comparison links. In the UI, hashes are truncated to 8 characters, and the full hash is displayed on mouseover.
11d61cb82e