[checkout] Avoid overriding existing branch merge config

commands/checkout.go

@@ -138,8 +138,11 @@ func transformCheckoutArgs(args *Args, pullRequest *github.PullRequest, newBranc
 			remote = project.GitURL("", "", true)
 			mergeRef = fmt.Sprintf("refs/heads/%s", pullRequest.Head.Ref)
 		}
-		args.After("git", "config", fmt.Sprintf("branch.%s.remote", newBranchName), remote)
-		args.After("git", "config", fmt.Sprintf("branch.%s.merge", newBranchName), mergeRef)
+
+		if mc, err := git.Config(fmt.Sprintf("branch.%s.merge", newBranchName)); err != nil || mc == "" {
+			args.After("git", "config", fmt.Sprintf("branch.%s.remote", newBranchName), remote)
+			args.After("git", "config", fmt.Sprintf("branch.%s.merge", newBranchName), mergeRef)
+		}
 	}
 	return
 }

features/checkout.feature

@@ -33,6 +33,33 @@ Feature: hub checkout <PULLREQ-URL>
     And "git checkout -f fixes -q" should be run
     And "fixes" should merge "refs/pull/77/head" from remote "origin"
 
+  Scenario: Avoid overriding existing merge configuration
+    Given the GitHub API server:
+      """
+      get('/repos/mojombo/jekyll/pulls/77') {
+        json :number => 77, :head => {
+          :ref => "fixes",
+          :repo => {
+            :owner => { :login => "mislav" },
+            :name => "jekyll",
+            :private => false
+          }
+        }, :base => {
+          :repo => {
+            :name => 'jekyll',
+            :html_url => 'https://github.com/mojombo/jekyll',
+            :owner => { :login => "mojombo" },
+          }
+        }, :maintainer_can_modify => false
+      }
+      """
+    Given I successfully run `git config branch.fixes.remote ORIG_REMOTE`
+    Given I successfully run `git config branch.fixes.merge custom/ref/spec`
+    When I successfully run `hub checkout https://github.com/mojombo/jekyll/pull/77`
+    Then "git fetch origin refs/pull/77/head:fixes" should be run
+    And "git checkout fixes" should be run
+    And "fixes" should merge "custom/ref/spec" from remote "ORIG_REMOTE"
+
   Scenario: Head ref matches default branch
     Given the GitHub API server:
       """