add explicit mapping of ConfigSourceType to precedence

Summary: Code review from D37169769 - its not easy to extend implicit precedence in thrift without unsafe reordering - instead lets map this ourselves in the code

Reviewed By: xavierd

Differential Revision: D48758164

fbshipit-source-id: faab964fae19cacc681628c5f31a6bc184a9a94f

eden/fs/config/eden_config.thrift

@@ -11,10 +11,7 @@ namespace py facebook.eden.eden_config
 namespace py3 eden.fs.config
 
 /**
- * Identifies the point of origin of a config setting. It is ordered from
- * low to high precedence. Higher precedence configuration values over-ride
- * lower precedence values. A config setting of CommandLine takes precedence
- * over all other settings.
+ * Identifies the point of origin of a config setting. Precedence is defined by ConfigSetting::getIdx()
  */
 enum ConfigSourceType {
   Default = 0,