Performance Test: Reuse tests-branch build if possible

When running the performance test suite we currently build three copies
of the plugin: one for each branch under test and also one for the branch
which provides the actual test suite.

In this patch we're checking first if one of the branches under test is
the same as that tests branch, and if so, re-using the built files from
that to avoid the additional approximately five minutes of building.

bin/plugin/commands/performance.js

@@ -260,12 +260,23 @@ async function runPerformanceTests( branches, options ) {
 		await runShellScript( 'mkdir ' + environmentDirectory );
 		await runShellScript( `cp -R ${ baseDirectory } ${ buildPath }` );
 
-		log( `        >> Fetching the ${ formats.success( branch ) } branch` );
-		// @ts-ignore
-		await SimpleGit( buildPath ).reset( 'hard' ).checkout( branch );
+		const fancyBranch = formats.success( branch );
 
-		log( `        >> Building the ${ formats.success( branch ) } branch` );
-		await runShellScript( 'npm ci && npm run build', buildPath );
+		if ( branch === options.testsBranch ) {
+			log(
+				`        >> Re-using the testing branch for ${ fancyBranch }`
+			);
+			await runShellScript(
+				`cp -R ${ performanceTestDirectory } ${ buildPath }`
+			);
+		} else {
+			log( `        >> Fetching the ${ fancyBranch } branch` );
+			// @ts-ignore
+			await SimpleGit( buildPath ).reset( 'hard' ).checkout( branch );
+
+			log( `        >> Building the ${ fancyBranch } branch` );
+			await runShellScript( 'npm ci && npm run build', buildPath );
+		}
 
 		await runShellScript(
 			'cp ' +