pull-request: detect default branch for project

This is usually "master" but for e.g. git-flow projects it's "develop".
GitHub has an API field for the default branch but I avoided having to
hit the API and instead read from the "head" of the origin remote.

  git rev-parse --symbolic-full-name origin
  #=> refs/remotes/origin/develop

The "head" of the remote seems to be set at clone time by default, and
can be manually set with:

  git remote set-head origin develop

The pull-request command now detects this and sets the default branch as
the pull request base instead of always assuming "master" at origin.

Fixes #154, closes #326

features/pull_request.feature

@@ -255,6 +255,20 @@ Feature: hub pull-request
     When I successfully run `hub pull-request -b develop -m message`
     Then the output should contain exactly "the://url\n"
 
+  Scenario: Implicit base by detecting main branch
+    Given the default branch for "origin" is "develop"
+    And I am on the "master" branch
+    Given the GitHub API server:
+      """
+      post('/repos/mislav/coral/pulls') {
+        assert :base => 'develop',
+               :head => 'mislav:master'
+        json :html_url => "the://url"
+      }
+      """
+    When I successfully run `hub pull-request -m message`
+    Then the output should contain exactly "the://url\n"
+
   Scenario: Explicit base with owner
     Given I am on the "master" branch
     Given the GitHub API server:

features/steps.rb

@@ -74,6 +74,16 @@ Given(/^I am on the "([^"]+)" branch(?: with upstream "([^"]+)")?$/) do |name, u
   run_silent %(git checkout --quiet -B #{name} --track #{upstream})
 end
 
+Given(/^the default branch for "([^"]+)" is "([^"]+)"$/) do |remote, branch|
+  empty_commit
+  ref_file = ".git/refs/remotes/#{remote}/#{branch}"
+  in_current_dir do
+    FileUtils.mkdir_p File.dirname(ref_file)
+    FileUtils.cp '.git/refs/heads/master', ref_file
+  end
+  run_silent %(git remote set-head #{remote} #{branch})
+end
+
 Given(/^I am in detached HEAD$/) do
   empty_commit
   empty_commit

lib/hub/context.rb

@@ -159,7 +159,10 @@ module Hub
       end
 
       def master_branch
-        Branch.new self, 'refs/heads/master'
+        if remote = origin_remote
+          default_branch = git_command("rev-parse --symbolic-full-name #{remote}")
+        end
+        Branch.new(self, default_branch || 'refs/heads/master')
       end
 
       def remotes

test/hub_test.rb

@@ -260,6 +260,7 @@ class HubTest < Test::Unit::TestCase
   def test_pullrequest_from_branch_tracking_local
     stub_branch('refs/heads/feature')
     stub_tracking('feature', 'refs/heads/master')
+    stub_command_output('rev-parse --symbolic-full-name origin', 'refs/remotes/origin/master')
 
     stub_request(:post, "https://api.github.com/repos/defunkt/hub/pulls").
       with(:body => {'base' => "master", 'head' => "tpw:feature", 'title' => "hereyougo" }).
@@ -274,6 +275,7 @@ class HubTest < Test::Unit::TestCase
     stub_repo_url('git@git.my.org:defunkt/hub.git')
     stub_branch('refs/heads/feature')
     stub_tracking_nothing('feature')
+    stub_command_output('rev-parse --symbolic-full-name origin', 'refs/remotes/origin/master')
     stub_command_output "rev-list --cherry-pick --right-only --no-merges origin/feature...", nil
     edit_hub_config do |data|
       data['git.my.org'] = [{'user'=>'myfiname', 'oauth_token' => 'FITOKEN'}]