SVN 1.9 uses (nonexistent) for new files, not (revision 0).
Review Request #7617 — Created Sept. 3, 2015 and submitted
Trivial fix:
rbt post
fails to upload a diff if it includes new files and SVN 1.9 is used.
Tested on 2.0.19 by uploading a diff using svn 1.9; the same patch applies cleanly to master branch.