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Autocompletion fails if there's only one mark, since the ls command
will not display the parent directory with the trailing colon.
Handling the single mark case separately and validating the symlink
explicitly, resolves the issue.

Change marks function and remove 'function' keyword as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22820224

Add _completemarks function as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22826540