commit_date: code cleanup and readme updates

[crazy-max]

follow-up #471 (comment)

@trim2 Fyi, I see in response schema that commit.committer for /repos/{owner}/{repo}/commits/{ref}: https://docs.github.com/en/rest/commits/commits?apiVersion=2022-11-28#get-a-commit

Can be null:

        "committer": {
          "anyOf": [
            {
              "type": "null"
            },
            {
              "title": "Git User",
              "description": "Metaproperties for Git author/committer information.",
              "type": "object",
              "properties": {
                "name": {
                  "type": "string",
                  "examples": [
                    "\"Chris Wanstrath\""
                  ]
                },
                "email": {
                  "type": "string",
                  "examples": [
                    "\"[email protected]\""
                  ]
                },
                "date": {
                  "type": "string",
                  "examples": [
                    "\"2007-10-29T02:42:39.000-07:00\""
                  ]
                }
              }
            }
          ]
        },

If that happens it would just return the current date. I think that's fine but we could improve that as follow-up. Probably warn if that happens.