Add .gitattribute assisted language detection to blame, diff and render (#17590)

Use check attribute code to check the assigned language of a file and send that in to
chroma as a hint for the language of the file.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/highlight/highlight_test.go

@@ -96,7 +96,7 @@ steps:
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			if got := File(tt.numLines, tt.fileName, []byte(tt.code)); !reflect.DeepEqual(got, tt.want) {
+			if got := File(tt.numLines, tt.fileName, "", []byte(tt.code)); !reflect.DeepEqual(got, tt.want) {
 				t.Errorf("File() = %v, want %v", got, tt.want)
 			}
 		})