Make the git download strategy default to :branch => "master"

Fixes Homebrew/homebrew#33437.

Library/Homebrew/download_strategy.rb

@@ -444,6 +444,8 @@ class GitDownloadStrategy < VCSDownloadStrategy
 
   def initialize name, resource
     super
+    @ref_type ||= :branch
+    @ref ||= "master"
     @shallow = resource.specs.fetch(:shallow) { true }
   end
 
@@ -473,11 +475,7 @@ class GitDownloadStrategy < VCSDownloadStrategy
   def stage
     dst = Dir.getwd
     @clone.cd do
-      if @ref_type and @ref
-        ohai "Checking out #@ref_type #@ref"
-      else
-        reset
-      end
+      ohai "Checking out #{@ref_type} #{@ref}"
       # http://stackoverflow.com/questions/160608/how-to-do-a-git-export-like-svn-export
       safe_system 'git', 'checkout-index', '-a', '-f', "--prefix=#{dst}/"
       checkout_submodules(dst) if submodules?
@@ -535,9 +533,7 @@ class GitDownloadStrategy < VCSDownloadStrategy
   end
 
   def update_repo
-    # Branches always need updated. The has_ref? check will only work if a ref
-    # has been specified; if there isn't one we always want an update.
-    if @ref_type == :branch || !@ref || !has_ref?
+    if @ref_type == :branch || !has_ref?
       quiet_safe_system 'git', 'fetch', 'origin'
     end
   end
@@ -547,24 +543,14 @@ class GitDownloadStrategy < VCSDownloadStrategy
     @clone.cd { update_submodules } if submodules?
   end
 
-  def checkout_args
-    ref = case @ref_type
-          when :branch, :tag, :revision then @ref
-          else `git symbolic-ref refs/remotes/origin/HEAD`.strip.split("/").last
-          end
-
-    %W{checkout -f #{ref}}
-  end
-
   def checkout
-    quiet_safe_system 'git', *checkout_args
+    quiet_safe_system "git", "checkout", "-f", @ref, "--"
   end
 
   def reset_args
     ref = case @ref_type
           when :branch then "origin/#@ref"
           when :revision, :tag then @ref
-          else "origin/HEAD"
           end
 
     %W{reset --hard #{ref}}