diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index 0684461..7dce71a 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -86,7 +86,7 @@ class MergeRequestDiff < ActiveRecord::Base
   # between target and source branches
   def unmerged_commits
     commits = if merge_request.for_fork?
-                Gitlab::Satellite::MergeAction.new(merge_request.author, merge_request).commits_between
+                compare_action.commits
               else
                 repository.commits_between(target_branch, source_branch)
               end
@@ -150,7 +150,7 @@ class MergeRequestDiff < ActiveRecord::Base
   # between target and source branches
   def unmerged_diffs
     diffs = if merge_request.for_fork?
-              Gitlab::Satellite::MergeAction.new(merge_request.author, merge_request).diffs_between_satellite
+              compare_action.diffs
             else
               Gitlab::Git::Diff.between(repository, source_branch, target_branch)
             end
@@ -165,4 +165,16 @@ class MergeRequestDiff < ActiveRecord::Base
   def repository
     merge_request.target_project.repository
   end
+
+  private
+
+  def compare_action
+    Gitlab::Satellite::CompareAction.new(
+      merge_request.author,
+      merge_request.target_project,
+      merge_request.target_branch,
+      merge_request.source_project,
+      merge_request.source_branch
+    )
+  end
 end
diff --git a/lib/gitlab/satellite/compare_action.rb b/lib/gitlab/satellite/compare_action.rb
new file mode 100644
index 0000000..c923bb9
--- /dev/null
+++ b/lib/gitlab/satellite/compare_action.rb
@@ -0,0 +1,53 @@
+module Gitlab
+  module Satellite
+    class CompareAction < Action
+      def initialize(user, target_project, target_branch, source_project, source_branch)
+        super user, target_project
+
+        @target_project, @target_branch = target_project, target_branch
+        @source_project, @source_branch = source_project, source_branch
+      end
+
+      # Only show what is new in the source branch compared to the target branch, not the other way around.
+      # The line below with merge_base is equivalent to diff with three dots (git diff branch1...branch2)
+      # From the git documentation: "git diff A...B" is equivalent to "git diff $(git-merge-base A B) B"
+      def diffs
+        in_locked_and_timed_satellite do |target_repo|
+          prepare_satellite!(target_repo)
+          update_satellite_source_and_target!(target_repo)
+          common_commit = target_repo.git.native(:merge_base, default_options, ["origin/#{@target_branch}", "source/#{@source_branch}"]).strip
+          #this method doesn't take default options
+          diffs = target_repo.diff(common_commit, "source/#{@source_branch}")
+          diffs = diffs.map { |diff| Gitlab::Git::Diff.new(diff) }
+          diffs
+        end
+      rescue Grit::Git::CommandFailed => ex
+        handle_exception(ex)
+      end
+
+      # Retrieve an array of commits between the source and the target
+      def commits
+        in_locked_and_timed_satellite do |target_repo|
+          prepare_satellite!(target_repo)
+          update_satellite_source_and_target!(target_repo)
+          commits = target_repo.commits_between("origin/#{@target_branch}", "source/#{@source_branch}")
+          commits = commits.map { |commit| Gitlab::Git::Commit.new(commit, nil) }
+          commits
+        end
+      rescue Grit::Git::CommandFailed => ex
+        handle_exception(ex)
+      end
+
+      private
+
+      # Assumes a satellite exists that is a fresh clone of the projects repo, prepares satellite for diffs
+      def update_satellite_source_and_target!(target_repo)
+        target_repo.remote_add('source', @source_project.repository.path_to_repo)
+        target_repo.remote_fetch('source')
+        target_repo.git.checkout(default_options({b: true}), @target_branch, "origin/#{@target_branch}")
+      rescue Grit::Git::CommandFailed => ex
+        handle_exception(ex)
+      end
+    end
+  end
+end
--
libgit2 0.21.2