Cache successful commits

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

git checkout HEAD~1 bin
run test/git-check-assertions.bats
assert_failure
assert_output --partial "file exists, but it was expected to be absent"
```

.gitignore

@@ -1 +1,2 @@
 /result
+.git-check-assertions-cache

README.md

@@ -73,6 +73,12 @@ You can technically assert that a command fails by writing `! ... || exit 1`, or
 
 I'm considering taking `bats-assert` as a dependency, but for now, this very minimal set of functions with a similar interface should get you on your way.
 
+## Cache of successful commits
+
+After a commit's assertions run successfully, `git-check-assertions` appends the commit hash to `.git-check-assertions-cache` in the repo root. On the next run, any commit listed there is skipped.
+
+Be sure to add `.git-check-assertions-cache` to your `.gitignore`.
+
 ## Examples of commit messages
 
 Assert that a commit builds:

bin/git-check-assertions

@@ -87,7 +87,22 @@ echo "Base: $base"
 echo "Commits to visit: ${#commits[@]}"
 echo
 
+declare -A cached_commits=()
+if [ -f .git-check-assertions-cache ]; then
+  while IFS= read -r cached_commit; do
+    if [ -n "$cached_commit" ]; then
+      cached_commits["$cached_commit"]=1
+    fi
+  done < .git-check-assertions-cache
+fi
+
 for commit_hash in "${commits[@]}"; do
+  if [ -n "${cached_commits[$commit_hash]+x}" ]; then
+    echo "Skipping $commit_hash (cached)"
+    echo
+    continue
+  fi
+
   echo "Checking out $commit_hash"
   git -c advice.detachedHead=false checkout -q "$commit_hash"
   commit_msg="$(git log -1 --format=%B "$commit_hash")"
@@ -107,6 +122,7 @@ for commit_hash in "${commits[@]}"; do
     fi
     restore
   fi
+  printf '%s\n' "$commit_hash" >>.git-check-assertions-cache
   echo
 done
 

test/git-check-assertions.bats

@@ -9,9 +9,14 @@ init_repo() {
   git init -b "$main_branch"
   git config user.email "john.doe@example.com"
   git config user.name "John Doe"
+
+  echo ".git-check-assertions-cache" >>.gitignore
+  git add .gitignore
+  git commit -m "add .git-check-assertions-cache to gitignore"
+
   touch readme
   git add readme
-  git commit -m "initial"
+  git commit -m "add readme"
 }
 
 setup() {
@@ -195,6 +200,57 @@ commit_with_assertion() {
   assert_size_zero readme
 }
 
+@test "should skip cached commits on subsequent runs" {
+  git checkout -b feature
+  commit_with_assertion "touch ../test"
+
+  run git-check-assertions
+
+  assert_success
+  assert_file_exists ../test
+  rm ../test
+
+  run git-check-assertions
+
+  assert_success
+  assert_file_not_exists ../test
+}
+
+@test "should not cache failed commits" {
+  git checkout -b feature
+  commit_with_assertion "touch ../test && exit 3"
+
+  run git-check-assertions
+
+  assert_failure
+  assert_file_exists ../test
+  rm ../test
+
+  run git-check-assertions
+
+  assert_failure
+  assert_file_exists ../test
+}
+
+@test "should skip cached commits when multiple commits are cached" {
+  git checkout -b feature
+  commit_with_assertion "touch ../test1"
+  commit_with_assertion "touch ../test2"
+
+  run git-check-assertions
+
+  assert_success
+  assert_file_exists ../test1
+  assert_file_exists ../test2
+  rm ../test1 ../test2
+
+  run git-check-assertions
+
+  assert_success
+  assert_file_not_exists ../test1
+  assert_file_not_exists ../test2
+}
+
 @test "assertions should run against the version of the code inside of the commit" {
   git checkout -b feature
   echo commit1 >readme