nix-prefetch-git: make it quiet

Some programs (e.g. nvfetcher) parse the output of nix-prefetch-git as
JSON. However, there is a pushd/popd command in nix-prefetch-git that
outputs the directory name, breaking the structure of the result JSON.
We suppressed it with `>/dev/null`.

Passingly fixes a mixuse of tab and spaces.

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

@@ -266,9 +266,9 @@ make_deterministic_repo(){
     else
         local dotgit_dir=".git"
     fi
-    pushd "$dotgit_dir"
+    pushd "$dotgit_dir" >/dev/null
         rm -rf logs/ hooks/ index FETCH_HEAD ORIG_HEAD refs/remotes/origin/HEAD config
-    popd
+    popd >/dev/null
     # Remove all remote branches.
     git branch -r | while read -r branch; do
         clean_git branch -rD "$branch"