[rubygems/rubygems] Clone rubygems from the local repo

It simplifies things and should avoid issues like the one we had where
the master branch had a bad `.gitattributes` file and changing it on a
PR would be disregarded.

In order for this to work, we need to make sure to fetch all tags from
the repository, so that they can properly be checked out later. This
does not apply to the case of testing against `RGV=..`, since no extra
cloning is needed there.

https://github.com/rubygems/rubygems/commit/d088d936b8

spec/bundler/support/rubygems_version_manager.rb

@@ -108,7 +108,7 @@ private
 
     unless rubygems_path.directory?
       rubygems_path.parent.mkpath
-      sys_exec("git clone https://github.com/rubygems/rubygems.git #{rubygems_path}")
+      sys_exec("git clone .. #{rubygems_path}", :dir => source_root)
     end
 
     rubygems_path