`fetchGit` has been modified a long time ago to use fetchTree, however,
we don't care about `lastModified` because we are not in a flake
context, this hack introduces a `git-locked` type of input that only
cares about `narHash` being present. This is needed to avoid fetching
the remote repo each time `fetchGit` is evaluated whith the result
present in the store.
Change-Id: I521c6fcccf8cf12945594f205d7fd4c8c2cf89e9
