Include repository cache files in the disk cache key. (#64)

Like BUILD file changes, changes to workspace dependencies can
significantly affect the contents of the disk cache. Prior to this
change, upreving dependencies wasn't itself sufficient to update the
disk cache, resulting in repeatedly rebuilding dependencies until some
subsequent commit happened to modify BUILD files.

config.js

@@ -139,6 +139,7 @@ module.exports = {
   diskCache: {
     enabled: diskCacheEnabled,
     files: [
+      ...repositoryCacheFiles,
       '**/BUILD.bazel',
       '**/BUILD'
     ],