mm(1): fixes bug with -a; mm.1, tests/mm.mk: changes to test regression in -a and -t

src/mm.rs

@@ -168,7 +168,7 @@ fn main() -> ExitCode {
 
 		let options = File::options()
 			/* don’t truncate if -t is specified, append if -a is specified */
-			.truncate(t)
+			.truncate(!a && t)
 			.append(a)
 			/* enable the ability to create and write to files */
 			.create(true)