feat: add internal reproducible git checksum builtin (#203)

Adds a new rego builtin `attest.internals.reproducible_git_checksum`.
This is needed for verifying DOI provenance, see
https://github.com/docker/doi-image-policy/blob/main/slsa.md#doi-build-reproducible-git-checksum.

We use https://github.com/go-git/go-git for as much of this as possible,
but it doesn't support the actual archive operation, so we shell out to
`git` for that.

There is some similar unexported code in bashbrew, and we should
probably be using the same code in the build process as we are here.
I'll create a follow-up ticket to sort that out.

policy/testdata/policies/test/git_checksum/git_checksum_test.rego

@@ -0,0 +1,26 @@
+package git_checksum_test
+
+import rego.v1
+
+test_reproducible_git_checksum if {
+	# test case from https://github.com/docker-library/meta/blob/5c3af85f2c735ea2b689271cb64ff38bcca28bec/builds.json
+	#  build id: e1dc43214da28419a105a665f994080e83093c6849fe2851344350b8c264afd1
+	# grab with `curl https://raw.githubusercontent.com/docker-library/meta/5c3af85f2c735ea2b689271cb64ff38bcca28bec/builds.json | jq '."e1dc43214da28419a105a665f994080e83093c6849fe2851344350b8c264afd1"'`
+
+	repo := "https://github.com/docker-library/busybox.git"
+	commit := "91f9975d4bb91d7c916ef74de77911d961ac9b75"
+	dir := "latest/glibc/amd64"
+	expected_checksum := "48d47b7ee1617a53291a76942cd240773fbb59daaa874007c6d16cb3125d63c2"
+
+	result := attest.internals.reproducible_git_checksum(repo, commit, dir)
+	actual_checksum := result.value
+	actual_checksum == expected_checksum
+
+	invalid_commit := "0000000000000000000000000000000000000000"
+	bad_commit_result := attest.internals.reproducible_git_checksum(repo, invalid_commit, dir)
+	contains(bad_commit_result.error, "failed to fetch")
+
+	invalid_dir := "not_a_real_dir"
+	bad_dir_result := attest.internals.reproducible_git_checksum(repo, commit, invalid_dir)
+	contains(bad_dir_result.error, "not a valid object name")
+}