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"
```

default.nix

@@ -7,6 +7,7 @@
   python3Packages,
   shellcheck-minimal,
   shfmt,
+  patchutils_0_4_2,
 }:
 
 let
@@ -42,6 +43,7 @@ stdenv.mkDerivation {
     python3Packages.flake8
     shellcheck-minimal
     shfmt
+    patchutils_0_4_2
   ];
 
   checkPhase = ''