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.

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/containerd/platforms v0.2.1
 	github.com/distribution/reference v0.6.0
 	github.com/docker-library/bashbrew v0.1.12
+	github.com/go-git/go-git/v5 v5.5.1
 	github.com/go-openapi/runtime v0.28.0
 	github.com/go-openapi/strfmt v0.23.0
 	github.com/google/go-containerregistry v0.20.2
@@ -40,9 +41,11 @@ require (
 	cloud.google.com/go/iam v1.2.1 // indirect
 	cloud.google.com/go/kms v1.20.0 // indirect
 	cloud.google.com/go/longrunning v0.6.1 // indirect
+	github.com/Microsoft/go-winio v0.6.2 // indirect
 	github.com/Microsoft/hcsshim v0.12.6 // indirect
 	github.com/OneOfOne/xxhash v1.2.8 // indirect
 	github.com/ProtonMail/go-crypto v1.0.0 // indirect
+	github.com/acomagu/bufpipe v1.0.3 // indirect
 	github.com/agnivade/levenshtein v1.2.0 // indirect
 	github.com/asaskevich/govalidator v0.0.0-20230301143203-a9d515a09cc2 // indirect
 	github.com/aws/aws-sdk-go-v2 v1.32.2 // indirect
@@ -76,9 +79,12 @@ require (
 	github.com/docker/distribution v2.8.3+incompatible // indirect
 	github.com/docker/docker-credential-helpers v0.8.1 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/fsnotify/fsnotify v1.7.0 // indirect
 	github.com/go-chi/chi v4.1.2+incompatible // indirect
+	github.com/go-git/gcfg v1.5.0 // indirect
+	github.com/go-git/go-billy/v5 v5.3.1 // indirect
 	github.com/go-ini/ini v1.67.0 // indirect
 	github.com/go-jose/go-jose/v4 v4.0.4 // indirect
 	github.com/go-logr/logr v1.4.2 // indirect
@@ -103,11 +109,14 @@ require (
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
 	github.com/hashicorp/go-retryablehttp v0.7.7 // indirect
 	github.com/hashicorp/hcl v1.0.1-vault-5 // indirect
+	github.com/imdario/mergo v0.3.16 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
 	github.com/jedisct1/go-minisign v0.0.0-20230811132847-661be99b8267 // indirect
 	github.com/jellydator/ttlcache/v3 v3.3.0 // indirect
 	github.com/jmespath/go-jmespath v0.4.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
+	github.com/kevinburke/ssh_config v1.2.0 // indirect
 	github.com/klauspost/compress v1.17.10 // indirect
 	github.com/letsencrypt/boulder v0.0.0-20240620165639-de9c06129bec // indirect
 	github.com/magiconair/properties v1.8.7 // indirect
@@ -120,6 +129,7 @@ require (
 	github.com/opencontainers/go-digest v1.0.0 // indirect
 	github.com/opentracing/opentracing-go v1.2.0 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
+	github.com/pjbgf/sha1cd v0.2.3 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/prometheus/client_golang v1.20.4 // indirect
@@ -130,10 +140,12 @@ require (
 	github.com/sagikazarmark/locafero v0.4.0 // indirect
 	github.com/sagikazarmark/slog-shim v0.1.0 // indirect
 	github.com/sassoftware/relic v7.2.1+incompatible // indirect
+	github.com/sergi/go-diff v1.3.1 // indirect
 	github.com/shibumi/go-pathspec v1.3.0 // indirect
 	github.com/sigstore/protobuf-specs v0.3.2 // indirect
 	github.com/sigstore/timestamp-authority v1.2.2 // indirect
 	github.com/sirupsen/logrus v1.9.3 // indirect
+	github.com/skeema/knownhosts v1.1.0 // indirect
 	github.com/sourcegraph/conc v0.3.0 // indirect
 	github.com/spf13/afero v1.11.0 // indirect
 	github.com/spf13/cast v1.6.0 // indirect
@@ -147,6 +159,7 @@ require (
 	github.com/titanous/rocacheck v0.0.0-20171023193734-afe73141d399 // indirect
 	github.com/transparency-dev/merkle v0.0.2 // indirect
 	github.com/vbatts/tar-split v0.11.5 // indirect
+	github.com/xanzy/ssh-agent v0.3.3 // indirect
 	github.com/xeipuuv/gojsonpointer v0.0.0-20190905194746-02993c407bfb // indirect
 	github.com/xeipuuv/gojsonreference v0.0.0-20180127040603-bd5ef7bd5415 // indirect
 	github.com/yashtewari/glob-intersection v0.2.0 // indirect
@@ -176,6 +189,7 @@ require (
 	google.golang.org/grpc v1.67.1 // indirect
 	google.golang.org/protobuf v1.35.1 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect
+	gopkg.in/warnings.v0 v0.1.2 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	gotest.tools/v3 v3.5.1 // indirect