Add subcommand for diffs

lib/xcodeproj/command.rb

@@ -4,8 +4,7 @@ module Xcodeproj
 
   class Command < CLAide::Command
     require 'xcodeproj/command/config_dump'
-    require 'xcodeproj/command/target_diff'
-    require 'xcodeproj/command/project_diff'
+    require 'xcodeproj/command/diff'
     require 'xcodeproj/command/show'
     require 'xcodeproj/command/sort'
 

lib/xcodeproj/command/diff.rb

@@ -0,0 +1,10 @@
+module Xcodeproj
+  class DiffCommand < Command
+    require 'xcodeproj/command/diff/project'
+    require 'xcodeproj/command/diff/target'
+
+    self.abstract_command = true
+    self.command = 'diff'
+    self.description = 'Shows the differences between Xcode objects.'
+  end
+end

lib/xcodeproj/command/project_diff.rb → lib/xcodeproj/command/diff/project.rb

@@ -1,6 +1,6 @@
 module Xcodeproj
-  class Command
-    class ProjectDiff < Command
+  class DiffCommand
+    class Project < DiffCommand
       self.summary = 'Shows the difference between two projects'
 
       self.description = summary + <<-EOS.gsub(/ {8}/, '')

lib/xcodeproj/command/target_diff.rb → lib/xcodeproj/command/diff/target.rb

@@ -1,6 +1,6 @@
 module Xcodeproj
-  class Command
-    class TargetDiff < Command
+  class DiffCommand
+    class Target < DiffCommand
       self.summary = 'Shows the difference between two targets'
 
       def self.options