Create Changelog when performing BundleInstall!

Keeps track of the current commit with a vundle_update tag before
perfoming an update and adds all commits pulled in the update to a
Changelog accessible via pressing 'u' after BundleInstall! completes.

autoload/vundle.vim

@@ -37,6 +37,8 @@ endif
 
 func! vundle#rc(...) abort
   let g:bundle_dir = len(a:000) > 0 ? expand(a:1, 1) : expand('$HOME/.vim/bundle', 1)
+  let g:updated_bundles = []
   let g:vundle_log = []
+  let g:vundle_changelog = ['Updated Bundles:']
   call vundle#config#init()
 endf