Use 3 way merge for merger.rb

Since --3way insists --index, git diff needs to compare with HEAD.

tool/merger.rb

@@ -204,7 +204,7 @@ class << Merger
     if svn_mode?
       command = %w[svn diff --diff-cmd=diff -x -upw]
     else
-      command = %w[git diff --color]
+      command = %w[git diff --color HEAD]
     end
     IO.popen(command + [file].compact, &:read)
   end
@@ -325,7 +325,7 @@ else
 
       message = "\n\n#{(patch[/^Subject: (.*)\n\ndiff --git/m, 1] || "Message not found for revision: #{git_rev}\n")}"
       puts '+ git apply'
-      IO.popen(['git', 'apply'], 'wb') { |f| f.write(patch) }
+      IO.popen(['git', 'apply', '--3way'], 'wb') { |f| f.write(patch) }
     else
       default_merge_branch = (%r{^URL: .*/branches/ruby_1_8_} =~ `svn info` ? 'branches/ruby_1_8' : 'trunk')
       svn_src = "#{Merger::REPOS}#{ARGV[1] || default_merge_branch}"