[Spec] Read the HEAD from the fixture repo

Don't break the spec everytime when comitting in the inner repo. That should ensure that we can keep the staging area in there clean.

spec/unit/installer/pod_source_installer_spec.rb

@@ -2,6 +2,8 @@ require File.expand_path('../../../spec_helper', __FILE__)
 
 module Pod
   describe Installer::PodSourceInstaller do
+    FIXTURE_HEAD = Dir.chdir(SpecHelper.fixture('banana-lib')) { `git rev-parse HEAD`.chomp }
+
     before do
       @spec = fixture_spec('banana-lib/BananaLib.podspec')
       @spec.source = { :git => SpecHelper.fixture('banana-lib') }
@@ -25,7 +27,7 @@ module Pod
           config.sandbox.store_head_pod('BananaLib')
           @spec.source = { :git => SpecHelper.fixture('banana-lib'), :tag => 'v1.0' }
           @installer.install!
-          @installer.specific_source[:commit].should == '9c7802033af588bed9dd5cb089bc8998a65bbd29'
+          @installer.specific_source[:commit].should == FIXTURE_HEAD
           pod_folder = config.sandbox.pod_dir('BananaLib')
           pod_folder.should.exist
         end
@@ -45,7 +47,7 @@ module Pod
           sources = @installer.sandbox.checkout_sources
           sources.should == { 'BananaLib' => {
             :git => SpecHelper.fixture('banana-lib'),
-            :commit => '9c7802033af588bed9dd5cb089bc8998a65bbd29' },
+            :commit => FIXTURE_HEAD },
           }
         end