feat: add commit digest in subject

[migmartri]

It adds sha1 digest of the git repository where the attestation is made.

This value is empty if the attestation happens outside of a repo

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "predicateType": "chainloop.dev/attestation/v0.2",
  "subject": [
    {
      "name": "chainloop.workflow.empty",
      "digest": {
        "sha256": "eec3f9e5bd477a5d95c4aea898d8ce05339e658a9634e090514746e9a2e8ca84"
      }
    },
    {
      "name": "git.head",
      "digest": {
        "sha1": "83d34125f5f6abe66c450fc1ed37ec630a7d4101"
      }
    }
  ],
  "predicate": {
    "metadata": {
      "name": "empty",
      "project": "empty",
      "team": "",
      "initializedAt": "2023-10-11T07:45:24.329348618Z",
      "finishedAt": "2023-10-11T09:45:40.019661234+02:00",
      "workflowRunID": "6888c2a8-9332-47d7-be37-8d344cb19358",
      "workflowID": "7e2f7529-1bef-4ba3-b3d2-297f96df7749"
    },
    "builder": {
      "id": "chainloop.dev/cli/dev@sha256:8aad02874ab3e15a45ac53199c8828aaa8c4367c328fb9bf38e13bf751e9392a"
    },
    "buildType": "chainloop.dev/workflowrun/v0.1",
    "runnerType": "RUNNER_TYPE_UNSPECIFIED",
    "materials": [
      {
        "digest": {
          "sha256": "0173a54b14c1f163c3015c9ddd32c781f6074a32986b7bcd5448a719f4e285aa"
        },
        "name": "main.go",
        "annotations": {
          "chainloop.material.cas": true,
          "chainloop.material.name": "foo",
          "chainloop.material.type": "ARTIFACT"
        }
      }
    ]
  }
}

Refs #373

[danlishka]

Please make sure we document how this repo gets found and that it happens during the attestation init.