We could validate the commit hash (40 characters):

export class CommitHash {
  value: string

  constructor(value: string) {
    // TODO: validation
    this.value = value
  }
  ...
}

And short commit hash (7 character):

export class ShortCommitHash implements Nullable {
  private value: string

  constructor(value: string) {
    // TODO: validation
    this.value = value
  }
  ...
}

Sample implementation: https://github.com/kensho-technologies/check-more-types/blob/5356e344fc64f8130ee1f17de293abda695cf015/src/git.js#L35-L39

We wanted to force to use always the 40-character version to avoid ambiguity, but SimpleGit commit returns the short version. We could use our SimpleGit wrapper to change that behaviour and use always long hashes.