Fix the case of unavailable fork in `checkout`

Fixes #1293

commands/checkout.go

@@ -100,7 +100,11 @@ func transformCheckoutArgs(args *Args) error {
 		newArgs = append(newArgs, "-b", newBranchName, "--track", remoteBranch)
 	} else {
 		if newBranchName == "" {
-			newBranchName = fmt.Sprintf("%s-%s", pullRequest.Head.Repo.Owner.Login, pullRequest.Head.Ref)
+			if pullRequest.Head.Repo == nil {
+				newBranchName = fmt.Sprintf("pr-%s", id)
+			} else {
+				newBranchName = fmt.Sprintf("%s-%s", pullRequest.Head.Repo.Owner.Login, pullRequest.Head.Ref)
+			}
 		}
 		refSpec = fmt.Sprintf("pull/%s/head:%s", id, newBranchName)
 		newArgs = append(newArgs, newBranchName)

features/checkout.feature

@@ -120,3 +120,23 @@ Feature: hub checkout <PULLREQ-URL>
     When I run `hub checkout https://github.com/mojombo/jekyll/pull/77 mycustombranch`
     Then "git fetch origin +refs/heads/fixes:refs/remotes/origin/fixes" should be run
     And "git checkout -b mycustombranch --track origin/fixes" should be run
+
+  Scenario: Unavailable fork
+    Given the GitHub API server:
+      """
+      get('/repos/mojombo/jekyll/pulls/77') {
+        json :head => {
+          :ref => "fixes",
+          :repo => nil
+        }, :base => {
+          :repo => {
+            :name => "jekyll",
+            :html_url => "https://github.com/mojombo/jekyll",
+            :owner => { :login => "mojombo" },
+          }
+        }
+      }
+      """
+    When I run `hub checkout https://github.com/mojombo/jekyll/pull/77`
+    Then "git fetch origin pull/77/head:pr-77" should be run
+    And "git checkout pr-77" should be run

github/client.go

@@ -107,7 +107,8 @@ type PullRequestSpec struct {
 }
 
 func (pr *PullRequest) IsSameRepo() bool {
-	return pr.Head.Repo.Name == pr.Base.Repo.Name &&
+	return pr.Head.Repo != nil &&
+		pr.Head.Repo.Name == pr.Base.Repo.Name &&
 		pr.Head.Repo.Owner.Login == pr.Base.Repo.Owner.Login
 }