Use default shell completion when there are no other options.

There are two calls of 'complete' with different options.  This is
workaround for old versions of Bash.  Both Git and Mercurial use
the same technique in their completion scripts.

To be honest I copied this from Mercurial completion script.
See commit fa9ae7df88a9 in Mercurial's repository
(url: http://selenic.com/hg/diff/fa9ae7df88a9/contrib/bash_completion).

Issue jekyll/jekyll#1905