Show base cache key hash with DEBUG=Metro:Transformer

Summary:
Add a `debug` handy when debugging cache key mismatches / unexpected cache misses, especially in cases where stepping in with a debugger is tricky, and noting that print-based debugging is impossible because any change to this source changes the cache key.

Changelog: Internal

Reviewed By: yungsters

Differential Revision: D74717070

fbshipit-source-id: 216ff7f15363aae8fd7ac6d31cdc18157abe3204

Author: robhogan

packages/metro/src/DeltaBundler/Transformer.js

@@ -20,6 +20,7 @@ import crypto from 'crypto';
 const getTransformCacheKey = require('./getTransformCacheKey');
 const WorkerFarm = require('./WorkerFarm');
 const assert = require('assert');
+const debug = require('debug')('Metro:Transformer');
 const fs = require('fs');
 const {Cache, stableHash} = require('metro-cache');
 const path = require('path');
@@ -69,7 +70,9 @@ class Transformer {
           transformerConfig: transformerOptions,
         });
 
-    this._baseHash = stableHash([globalCacheKey]).toString('binary');
+    const baseHashBuffer = stableHash([globalCacheKey]);
+    this._baseHash = baseHashBuffer.toString('binary');
+    debug('Base hash: %s', baseHashBuffer.toString('hex'));
   }
 
   async transformFile(