nix-prefetch-git is either run as part of a build, usually sandboxed,
or outside a build, unsandboxed, to prefetch something that will later
be used in a build. It's important that the latter use produces
hashes that can be reproduced by the former.
One way that they can differ is if the user's git config does
something that changes the result of git clone. I ran into this,
because my global git config automatically enables git-lfs, whereas
nix-prefetch-git otherwise only uses git-lfs if specifically
requested. This led to very confusing hash mismatches.
7d40fbbc04