vim_configurable: change source to mercurial and update to 7.3-999

Vim release is rarely updated, but it receives official patches from time to time. Currently it is at patch 999(coincidence?). 

Well some vim plugins don't work with unpatched vim, and there isn't any way to get archive of all patches, you can just download every patch separately and apply all of them.

But! They also have source on mercurial repo, with tags for every patch, so we why not use that ;)