Optimize repos for size when preparing for nodecache

We currently run 'git gc' on repos before creating the tarball for the
node cache. On busy repos this is not enough to cut down the size of
the repo. With more aggressive pruning and packing one of our repos in
the cache got down from 6GB to 1.8GB.

Change-Id: I7ef428c25fde713efabd966d59b5bd97ec248d7e

roles/git-prepare-nodecache/tasks/main.yaml

@@ -19,7 +19,7 @@
     - skip_ansible_lint
 
 - name: Run git garbage collection
-  command: git gc
+  command: git gc --prune=now --aggressive
   args:
     chdir: "{{ git_cache_root }}/{{ item.canonical_name }}"
   with_items: "{{ zuul.projects.values() | list }}"