fix that Git#checkout will work with :new_branch

this small fix, will makes Git#checkout work if :new_branch is given as an argument, like

git = Git.open 'path/to/repo'
git.checkout 'branchName', :new_branch => true
This is syntactically invalid, the last `<<` must be removed.

Anyway, based on the PR comments some refactor should be done in order to make it backwards compatible.