When vundle installed as git submodule, git create file '.git' instead of directory '.git'. And vundle cannot update by self :(
Instead checking for specific text in the output of the git commands, determine the status of the bundle by comparing the sha of HEAD before and after the git pull command.
If no errors exist, the status-line will inform users how to view the Changelog after BundleUpdate! is completed.
Distinction between `new` and `updated` simplifies the process of adding a bundle to the updated_bundle list.