diff --git a/Library/Homebrew/cmd/update.rb b/Library/Homebrew/cmd/update.rb
index 5addb4375d..85cc3014b5 100644
--- a/Library/Homebrew/cmd/update.rb
+++ b/Library/Homebrew/cmd/update.rb
@@ -172,7 +172,7 @@ class Updater
     map = Hash.new{ |h,k| h[k] = [] }
 
     if initial_revision && initial_revision != current_revision
-      `git diff-tree -r --name-status --diff-filter=AMD #{initial_revision} #{current_revision}`.each_line do |line|
+      diff.each_line do |line|
         status, path = line.split
         map[status.to_sym] << repository.join(path)
       end
@@ -187,6 +187,10 @@ class Updater
     `git rev-parse -q --verify HEAD`.chomp
   end
 
+  def diff
+    `git diff-tree -r --name-status --diff-filter=AMD #{initial_revision} #{current_revision}`
+  end
+
   def `(cmd)
     out = super
     if $? && !$?.success?
diff --git a/Library/Homebrew/test/test_updater.rb b/Library/Homebrew/test/test_updater.rb
index 28ce48ad0b..0806b3d7c8 100644
--- a/Library/Homebrew/test/test_updater.rb
+++ b/Library/Homebrew/test/test_updater.rb
@@ -4,6 +4,8 @@ require 'yaml'
 
 class UpdaterTests < Homebrew::TestCase
   class UpdaterMock < ::Updater
+    attr_accessor :diff
+
     def initialize(*)
       super
       @outputs = Hash.new { |h, k| h[k] = [] }
@@ -51,12 +53,12 @@ class UpdaterTests < Homebrew::TestCase
 
   def perform_update(diff_output="")
     HOMEBREW_REPOSITORY.cd do
+      @updater.diff = diff_output
       @updater.in_repo_expect("git checkout -q master")
       @updater.in_repo_expect("git rev-parse -q --verify HEAD", "1234abcd")
       @updater.in_repo_expect("git config core.autocrlf false")
       @updater.in_repo_expect("git pull -q origin refs/heads/master:refs/remotes/origin/master")
       @updater.in_repo_expect("git rev-parse -q --verify HEAD", "3456cdef")
-      @updater.in_repo_expect("git diff-tree -r --name-status --diff-filter=AMD 1234abcd 3456cdef", diff_output)
       @updater.pull!
       @report.update(@updater.report)
     end