phabricator: add the uploadfile function This is needed to be able to submit binary files such as images in commits (and also non-UTF-8 text files). One of the primary disadvantages of the current use of createrawdiff is that Phabricator simply drops any binary diffs included in the patch, but not the files, so if such a diff is then committed it corrupts any binary files in it. Differential Revision: https://phab.mercurial-scm.org/D7047