nix-prefetch-git: run single-threaded 'git repack'

Without this, the generated pack files are non-deterministic.

I didn't notice this issue in my earlier testing, because my test repo
had too few commits for the thread scheduling to take effect. (Test repo
had about 10 commits.)

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

···
         fi
     done
 
-    # Do a full repack, for determinism.
-    # Repack does not add unreferenced objects to a pack file.
+    # Do a full repack. Must run single-threaded, or else we loose determinism.
+    git config pack.threads 1
     git repack -A -d -f
+    rm -f .git/config
 
     # Garbage collect unreferenced objects.
     git gc --prune=all