Fix duplicates and unneeded entries in mp compile function.
* helm-match-plugin.el (helm-compile-source--match-plugin): Do it.
* helm-plugin.el: Fix header-line attr, the plugin shouldn't return nil.
* helm.el (helm-candidates-in-buffer): Make source arg optional.
* helm-buffers.el (helm-revert-buffer): Check if buffer file exists.
* helm-files.el: Fix backslash expansion on Windows (#599).