Add GitInfo to Site for site-wide commit info

[BartG95]

For example, I want to link to the git repo containing the source code of my website. But the source might have progressed after the latest build/deploy, so I can't just link to the main branch (well, I can, but the main branch does not necessarily has the source code of the current build/deploy). Instead I want to link to the repo at a specific commit, specifically the commit we were at while building/deploying.

For the time being, I work around this by wrapping hugo in a Makefile, that also creates a 'commit_id' file in my assets folder containing the output of git rev-parse --verify HEAD, and then using that via os.ReadFile. It works fine, but it is far from elegant.