[Sources] Use `git pull` with the `--ff-only` flag

This fixes issue where since we were using `--no-commit`, git was
starting a merge but not committing it. Then subsequent `git pull`'s would
result in the following error from git:

> You have not concluded your merge (MERGE_HEAD exists).
> Please, commit your changes before you can merge.

This commit will mean that `--ff-only` is passed to `git pull`  which means
it wont rebase or merge. It will only allow a fast forward to progress
with the pull.

Fixes #2024

lib/cocoapods/sources_manager.rb

@@ -153,7 +153,7 @@ module Pod
           UI.section "Updating spec repo `#{source.name}`" do
             Dir.chdir(source.data_provider.repo) do
               begin
-                output = git!("pull --no-rebase --no-commit")
+                output = git!("pull --ff-only")
                 UI.puts output if show_output && !config.verbose?
               rescue Informative => e
                 raise Informative, 'An error occurred while performing ' \

spec/unit/sources_manager_spec.rb

@@ -116,20 +116,13 @@ module Pod
         set_up_test_repo_for_update
 
         SourcesManager.update(test_repo_path.basename.to_s, true)
-        UI.output.should.match /Already up-to-date/
+        UI.output.should.match /is up to date/
       end
 
-      it "is robust against user settings for git repos" do
+      it "uses the only fast forward git option" do
         set_up_test_repo_for_update
 
-        SourcesManager.expects(:git!).with() { |options| options.should.match /--no-rebase/ }
-        SourcesManager.update(test_repo_path.basename.to_s, true)
-      end
-
-      it "doesn't allow merge commits" do
-        set_up_test_repo_for_update
-
-        SourcesManager.expects(:git!).with() { |options| options.should.match /--no-commit/ }
+        SourcesManager.expects(:git!).with() { |options| options.should.match /--ff-only/ }
         SourcesManager.update(test_repo_path.basename.to_s, true)
       end