mbbill/undotree — pull request #29 by inkarkat (2014-05-16 08:53:05+00:00)
Title: Add canonical include guard.

I `:runtime` the plugin early in my .vimrc to check whether it is supported, and then the plugin (and especially the `:autocmds`) gets defined again when Vim sources the regular `plugin/undotree.vim`.
Add a canonical include guard, and allow to check whether the plugin is actually enabled by using a different guard value.