Use git commit hash as part of the cache key

avoids issues with model caching being incompatible across versions

lib/mastodon/redis_config.rb

@@ -24,7 +24,7 @@ setup_redis_env_url
 setup_redis_env_url(:cache, false)
 
 namespace       = ENV.fetch('REDIS_NAMESPACE', nil)
-cache_namespace = namespace ? namespace + '_cache' : 'cache'
+cache_namespace = [namespace, 'cache', `git rev-parse --short HEAD`.strip].compact.join('_')
 
 REDIS_CACHE_PARAMS = {
   expires_in: 10.minutes,