use relative ID for more readable message

src/ast/nodes/ThisExpression.js

@@ -1,5 +1,6 @@
 import Node from '../Node.js';
 import getLocation from '../../utils/getLocation.js';
+import relativeId from '../../utils/relativeId.js';
 
 const warning = `The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten. See https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined for more information`;
 
@@ -11,7 +12,7 @@ export default class ThisExpression extends Node {
 			this.alias = this.module.context;
 			if ( this.alias === 'undefined' ) {
 				const { line, column } = getLocation( this.module.code, this.start );
-				const detail = `${this.module.id} (${line}:${column + 1})`; // use one-based column number convention
+				const detail = `${relativeId( this.module.id )} (${line}:${column + 1})`; // use one-based column number convention
 				this.module.bundle.onwarn( `${detail} ${warning}` );
 			}
 		}

test/function/warn-on-top-level-this/_config.js

@@ -1,7 +1,6 @@
 const assert = require( 'assert' );
 
 module.exports = {
-	solo: true,
 	description: 'warns on top-level this (#770)',
 	warnings: warnings => {
 		assert.deepEqual( warnings, [