Override the `manifest` method in the Gem plugin so that the list of filese to package is provided from git and its include / exclude rules.
This method uses the `git ls-files` command to list the files that should be included in the packaging process for the project.
Returns an array of filenames as Strings.
# File lib/bones/plugins/git.rb, line 110 def manifest %x{git ls-files}.split end