Add partial change example

```git-check-assertions
sed -i 's/HEAD~/HEAD/' test/git-check-assertions.bats
run test/git-check-assertions.bats
assert_failure
assert_output --partial "Expected command to fail, but it succeeded."
```

test/git-check-assertions.bats

@@ -113,6 +113,22 @@ commit_with_assertion() {
   assert_success
 }
 
+@test "you can use git checkout in an assertion to assert against a mixed version of the code" {
+  git checkout -b feature
+  echo hello >src
+  echo 'grep hello src' >tests
+  git add src tests
+  git commit -m 'add src and tests'
+  echo goodbye >src
+  echo 'grep goodbye src' >tests
+  git add src tests
+  commit_with_assertion $'git checkout HEAD~ src\nrun bash tests\nassert_failure'
+
+  run git-check-assertions
+
+  assert_success
+}
+
 @test "assert_success should succeed if the executed command succeeded" {
   git checkout -b feature
   commit_with_assertion $'run exit 0\nassert_success'