Provide "require signed commits" on github_branch_protection

This commit provides changing the toggle to require signed commits on
branch protection.

The [API for signed commits](https://developer.github.com/v3/repos/branches/#get-required-signatures-of-protected-branch)
is separate endpoint from the branch protection url, and does not come on the same payload.
This means that we need to make an extra request to read and modify it.

This changes requires `go-github` at least v19.

Need:
 - [ ] Update `go-github` version: https://github.com/terraform-providers/terraform-provider-github/pull/207
 - [ ] Provide default required reviewers count to avoid GitHub update error: https://github.com/terraform-providers/terraform-provider-github/pull/211
Closes https://github.com/terraform-providers/terraform-provider-github/issues/87

github/resource_github_branch_protection.go

@@ -130,6 +130,11 @@ func resourceGithubBranchProtection() *schema.Resource {
 				Optional: true,
 				Default:  false,
 			},
+			"require_signed_commits": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				Default:  false,
+			},
 			"etag": {
 				Type:     schema.TypeString,
 				Computed: true,
@@ -165,6 +170,10 @@ func resourceGithubBranchProtectionCreate(d *schema.ResourceData, meta interface
 
 	d.SetId(buildTwoPartID(&repoName, &branch))
 
+	if err = requireSignedCommitsUpdate(d, meta); err != nil {
+		return err
+	}
+
 	return resourceGithubBranchProtectionRead(d, meta)
 }
 
@@ -189,6 +198,9 @@ func resourceGithubBranchProtectionRead(d *schema.ResourceData, meta interface{}
 	if err != nil {
 		if ghErr, ok := err.(*github.ErrorResponse); ok {
 			if ghErr.Response.StatusCode == http.StatusNotModified {
+				if err := requireSignedCommitsRead(d, meta); err != nil {
+					return fmt.Errorf("Error setting signed commit restriction: %v", err)
+				}
 				return nil
 			}
 			if ghErr.Response.StatusCode == http.StatusNotFound {
@@ -219,6 +231,10 @@ func resourceGithubBranchProtectionRead(d *schema.ResourceData, meta interface{}
 		return fmt.Errorf("Error setting restrictions: %v", err)
 	}
 
+	if err := requireSignedCommitsRead(d, meta); err != nil {
+		return fmt.Errorf("Error setting signed commit restriction: %v", err)
+	}
+
 	return nil
 }
 
@@ -262,6 +278,10 @@ func resourceGithubBranchProtectionUpdate(d *schema.ResourceData, meta interface
 
 	d.SetId(buildTwoPartID(&repoName, &branch))
 
+	if err = requireSignedCommitsUpdate(d, meta); err != nil {
+		return err
+	}
+
 	return resourceGithubBranchProtectionRead(d, meta)
 }
 
@@ -326,6 +346,56 @@ func flattenAndSetRequiredStatusChecks(d *schema.ResourceData, protection *githu
 	return d.Set("required_status_checks", []interface{}{})
 }
 
+func requireSignedCommitsRead(d *schema.ResourceData, meta interface{}) error {
+	client := meta.(*Organization).client
+
+	repoName, branch, err := parseTwoPartID(d.Id())
+	if err != nil {
+		return err
+	}
+	orgName := meta.(*Organization).name
+
+	ctx := context.WithValue(context.Background(), ctxId, d.Id())
+	if !d.IsNewResource() {
+		ctx = context.WithValue(ctx, ctxEtag, d.Get("etag").(string))
+	}
+
+	log.Printf("[DEBUG] Reading branch protection signed commit status: %s/%s (%s)", orgName, repoName, branch)
+	signedCommitStatus, _, err := client.Repositories.GetSignaturesProtectedBranch(ctx,
+		orgName, repoName, branch)
+	if err != nil {
+		log.Printf("[WARN] Not able to read signature protection: %s/%s (%s)", orgName, repoName, branch)
+		return nil
+	}
+
+	return d.Set("require_signed_commits", signedCommitStatus.Enabled)
+}
+
+func requireSignedCommitsUpdate(d *schema.ResourceData, meta interface{}) (err error) {
+	requiredSignedCommit := d.Get("require_signed_commits").(bool)
+	client := meta.(*Organization).client
+
+	repoName, branch, err := parseTwoPartID(d.Id())
+	if err != nil {
+		return err
+	}
+	orgName := meta.(*Organization).name
+
+	ctx := context.WithValue(context.Background(), ctxId, d.Id())
+	if !d.IsNewResource() {
+		ctx = context.WithValue(ctx, ctxEtag, d.Get("etag").(string))
+	}
+
+	if requiredSignedCommit {
+		log.Printf("[DEBUG] Enabling branch protection signed commit: %s/%s (%s) - $s", orgName, repoName, branch)
+		_, _, err = client.Repositories.RequireSignaturesOnProtectedBranch(ctx, orgName, repoName, branch)
+	} else {
+		log.Printf("[DEBUG] Removing branch protection signed commit: %s/%s (%s) - $s", orgName, repoName, branch)
+		_, err = client.Repositories.OptionalSignaturesOnProtectedBranch(ctx, orgName, repoName, branch)
+	}
+	return err
+}
+
 func flattenAndSetRequiredPullRequestReviews(d *schema.ResourceData, protection *github.Protection) error {
 	rprr := protection.RequiredPullRequestReviews
 	if rprr != nil {