Describe the bug

.all-contributorsrc files shouldn't need to include commitConvention or repoType properties. They both have default values as documented on https://allcontributors.org/docs/en/bot/configuration:

• commitConvention: "angular"
• repoType: "github"

But, running npx all-contributors-cli add ... is adding commitConvention or repoType properties with their default values to .all-contributorsrc files that don't have them explicitly.

 {
 	"contributors": [ ... ],
 	"projectName": "repros-example",
 	"projectOwner": "JoshuaKGoldberg",
+	"repoType": "github",
+	"commitConvention": "angular"
 }

To Reproduce

See https://github.com/JoshuaKGoldberg/repros/tree/all-contributors-cli-add-field-defaults.

1. Create a repo with a minimal .all-contributorsrc
2. Run npx all-contributors-cli add JoshuaKGoldberg doc

Expected behavior

The .all-contributorsrc file shouldn't have commitConvention or repoType added, since they have documented defaults.

Additional context

I filed a bunch of feature requests over on https://github.com/all-contributors/all-contributors for more default-friendly behavior. all-contributors/allcontributors.org#808 is something of a hub for them.