[checkout] Handle case when PR matches the current branch

commands/checkout.go

@@ -115,8 +115,16 @@ func transformCheckoutArgs(args *Args, pullRequest *github.PullRequest, newBranc
 		}
 		newArgs = append(newArgs, newBranchName)
 
+		b, errB := repo.CurrentBranch()
+		isCurrentBranch := errB == nil && b.ShortName() == newBranchName
+
 		ref := fmt.Sprintf("refs/pull/%d/head", pullRequest.Number)
-		args.Before("git", "fetch", baseRemote.Name, fmt.Sprintf("%s:%s", ref, newBranchName))
+		if isCurrentBranch {
+			args.Before("git", "fetch", baseRemote.Name, ref)
+			args.After("git", "merge", "--ff-only", "FETCH_HEAD")
+		} else {
+			args.Before("git", "fetch", baseRemote.Name, fmt.Sprintf("%s:%s", ref, newBranchName))
+		}
 
 		remote := baseRemote.Name
 		mergeRef := ref

features/checkout.feature

@@ -251,6 +251,35 @@ Feature: hub checkout <PULLREQ-URL>
     And "git checkout -f fixes -q" should be run
     And "fixes" should merge "refs/heads/fixes" from remote "git@github.com:mislav/jekyll.git"
 
+  Scenario: Modifiable fork into current branch
+    Given the GitHub API server:
+      """
+      get('/repos/mojombo/jekyll/pulls/77') {
+        json :number => 77, :head => {
+          :ref => "fixes",
+          :repo => {
+            :owner => { :login => "mislav" },
+            :name => "jekyll",
+            :html_url => "https://github.com/mislav/jekyll.git",
+            :private => false
+          },
+        }, :base => {
+          :repo => {
+            :name => 'jekyll',
+            :html_url => 'https://github.com/mojombo/jekyll',
+            :owner => { :login => "mojombo" },
+          }
+        }, :maintainer_can_modify => true
+      }
+      """
+    And I am on the "fixes" branch
+    And there is a git FETCH_HEAD
+    When I successfully run `hub checkout https://github.com/mojombo/jekyll/pull/77`
+    Then "git fetch origin refs/pull/77/head" should be run
+    And "git checkout fixes" should be run
+    And "git merge --ff-only FETCH_HEAD" should be run
+    And "fixes" should merge "refs/heads/fixes" from remote "git@github.com:mislav/jekyll.git"
+
   Scenario: Modifiable fork with HTTPS
     Given the GitHub API server:
       """