nix-prefetch-git: fix printing of revision

If the user explictly gives a ref such as "refs/heads/master", `git
rev-parse` failed because we only checked out the `fetchgit`
branch. Now, we also try `git rev-parse fetchgit` if the first call
fails, which fixes the issue.

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -217,7 +217,7 @@ clone_user_rev() {
             fi;;
     esac
 
-    echo "git revision is $(cd $dir && git rev-parse $rev)"
+    echo "git revision is $(cd $dir && (git rev-parse $rev 2> /dev/null || git rev-parse refs/heads/fetchgit) | tail -n1)"
 
     # Allow doing additional processing before .git removal
     eval "$NIX_PREFETCH_GIT_CHECKOUT_HOOK"