Review Bot was attempting to apply patches to symlinks and to binary

files, but had no means of doing so. For the kinds of tools that are run

by Review Bot, we shouldn't usually care much about these types of

changes.
An argument can be made for symlinks, but we're currently unable to

guarantee we'll apply these right, and symlink target support hasn't

been available until Review Board 4.0.6 anyway.
This change skips these file types, and skips unknown FileDiff status

codes, in order to minimize the kinds of changes that need to be patched

to the tree or passed to tools.

Relevant unit tests pass.