Make sources in sourcemap relative to map dir instead of root when no…

… prior sourcemap is available

lib/process-file.js

@@ -59,6 +59,7 @@ module.exports = async function processFile(inFile, outFile, relativePath, outDi
 
     if (options.sourceMap) {
       let newSourceMap = JSON.parse(result.map);
+      let pathLib = path;
 
       newSourceMap.sources = newSourceMap.sources.map(function(path) {
         if (path === relativePath) {
@@ -76,7 +77,7 @@ module.exports = async function processFile(inFile, outFile, relativePath, outDi
           // In broccoli-terser-sourcemap we know in this case we are generating
           // sourcemap for the file we are processing, changing 0 to the actual
           // file gives us the correct source.
-          return relativePath;
+          return pathLib.basename(relativePath);
         }
         return path;
       });