Merge pull request #185005 from amjoseph-nixpkgs/pr/fetchgit/fix-comment

/* NOTE:

   fetchgit has one problem: git fetch only works for refs.

   This is because fetching arbitrary (maybe dangling) commits may be a security risk

   This is because fetching arbitrary (maybe dangling) commits creates garbage collection risks

   and checking whether a commit belongs to a ref is expensive. This may

   change in the future when some caching is added to git (?)

   Usually refs are either tags (refs/tags/*) or branches (refs/heads/*)