Ignore mutants.out* in .gitignore (payjoin#603)

On each run, any existing mutants.out is renamed to mutants.out.old, and
any existing mutants.out.old is deleted. This change ensures that
mutants.out.old is also ignored.

Author: spacebear21

.gitignore

@@ -4,4 +4,4 @@ target
 *payjoin.sled
 Cargo.lock
 .vscode
-mutants.out
+mutants.out*