[rubygems/rubygems] Add debugging message to `Bundler::Source::Git#specs`

It's not obvious that the git source actually reinstalls a proper
checkout of the repository on usage if necessary. This adds a debugging
message to log that, just like it's done when explicitly installing the
source.

https://github.com/rubygems/rubygems/commit/04e8e42a43

lib/bundler/source/git.rb

@@ -191,6 +191,7 @@ module Bundler
 
         if requires_checkout? && !@copied
           fetch
+          Bundler.ui.debug "  * Checking out revision: #{ref}"
           git_proxy.copy_to(install_path, submodules)
           serialize_gemspecs_in(install_path)
           @copied = true