Fix `hub browse` on detached HEAD

Fixes #588

features/browse.feature

@@ -114,6 +114,18 @@ Feature: hub browse
     When I successfully run `hub browse`
     Then "open https://github.com/mislav/dotfiles/tree/baz/qux/moo" should be run
 
+  Scenario: No branch
+    Given I am in "git://github.com/mislav/dotfiles.git" git repo
+    And I am in detached HEAD
+    When I successfully run `hub browse`
+    Then "open https://github.com/mislav/dotfiles" should be run
+
+  Scenario: No branch to pulls
+    Given I am in "git://github.com/mislav/dotfiles.git" git repo
+    And I am in detached HEAD
+    When I successfully run `hub browse -- pulls`
+    Then "open https://github.com/mislav/dotfiles/pulls" should be run
+
   Scenario: Wiki repo
     Given I am in "git://github.com/defunkt/hub.wiki.git" git repo
     When I successfully run `hub browse`

lib/hub/commands.rb

@@ -688,7 +688,7 @@ module Hub
           project = local_repo.main_project
         else
           # $ hub browse
-          prefer_upstream = current_branch.master?
+          prefer_upstream = current_branch && current_branch.master?
           branch, project = remote_branch_and_project(method(:github_user), prefer_upstream)
           branch ||= master_branch
         end