Closes #707. bundle package removes .gem when source is changed to git

lib/bundler/runtime.rb

@@ -101,7 +101,7 @@ def prune_cache
         spec = Gem::Format.from_file_by_path(path).spec
 
         resolve.any? do |s|
-          s.name == spec.name && s.version == spec.version
+          s.name == spec.name && s.version == spec.version && !s.source.is_a?(Bundler::Source::Git)
         end
       end
 

spec/cache/gems_spec.rb

@@ -153,6 +153,20 @@
       cached_gem("activesupport-2.3.2").should_not exist
     end
 
+    it "removes .gems when gem changes to git source" do
+      build_git "rack"
+
+      install_gemfile <<-G
+        source "file://#{gem_repo2}"
+        gem "rack", :git => "#{lib_path("rack-1.0")}"
+        gem "actionpack"
+      G
+      cached_gem("rack-1.0.0").should_not exist
+      cached_gem("actionpack-2.3.2").should exist
+      cached_gem("activesupport-2.3.2").should exist
+    end
+
+
     it "doesn't remove gems that are for another platform" do
       simulate_platform "java" do
         install_gemfile <<-G