markup: microoptimise for many short filenames in directory (#1534)

* markup: microoptimise for many short filenames in directory

Move strings.ToLower() after the early-return length check. This is a safe operation in all cases and should slightly improve directory listing performance when a directory contains many thousands of files with short filenames.

* markup: expand test cases for IsReadmeFile()

modules/markup/markup_test.go

@@ -25,6 +25,7 @@ func TestMisc_IsReadmeFile(t *testing.T) {
 		"abcdefg",
 		"abcdefghijklmnopqrstuvwxyz",
 		"test.md.test",
+		"readmf",
 	}
 
 	for _, testCase := range trueTestCases {