Evaluate embedded environment variables in git core.editor

Fixes #430

lib/hub/context.rb

@@ -437,6 +437,7 @@ module Hub
     def git_editor
       # possible: ~/bin/vi, $SOME_ENVIRONMENT_VARIABLE, "C:\Program Files\Vim\gvim.exe" --nofork
       editor = git_command 'var GIT_EDITOR'
+      editor.gsub!(/\$(\w+|\{\w+\})/) { ENV[$1.tr('{}', '')] }
       editor = ENV[$1] if editor =~ /^\$(\w+)$/
       editor = File.expand_path editor if (editor =~ /^[~.]/ or editor.index('/')) and editor !~ /["']/
       # avoid shellsplitting "C:\Program Files"

test/context_test.rb

@@ -50,6 +50,20 @@ class ContextTest < Test::Unit::TestCase
     end
   end
 
+  def test_editor_with_embedded_env_variable
+    stub_command_output 'var GIT_EDITOR', '$EDITOR -w'
+    with_env('EDITOR', 'subl') do
+      assert_equal %w'subl -w', context.git_editor
+    end
+  end
+
+  def test_editor_with_curly_brackets_embedded_env_variable
+    stub_command_output 'var GIT_EDITOR', 'my${EDITOR}2 -w'
+    with_env('EDITOR', 'subl') do
+      assert_equal %w'mysubl2 -w', context.git_editor
+    end
+  end
+
   private
 
   def stub_command_output(cmd, value)