Fix changes sometimes disappearing entirely

```git-check-assertions
run test/mechanicaldiff.bats
assert_success

git checkout HEAD~ bin
run test/mechanicaldiff.bats
assert_failure
assert_output --partial "not ok 9 keeps matching change with an extra removed line"
assert_output --partial "not ok 10 keeps matching change with an extra added line"
```

flake.nix

@@ -26,6 +26,7 @@
             pkgs.python3Packages.flake8
             pkgs.shellcheck
             pkgs.shfmt
+            pkgs.patchutils_0_4_2
           ];
         };
       }