Bugfix: fix integration with git command -h
Bugfix: fix integration with git command -h

"git vendor -h" produces an unhelpful message:

  $ git vendor -h
  usage: git vendor

Move some code so a proper usage message will be issued.

Note that "git vendor --help" is processed by the "git" binary (not
"git-vendor") and thus is unaffected by this change.

brettlangdon/git-vendor