rbt patch can now write patches to a local file by passing

--write=<filename>. This works just like --print with a redirect to

a file, but avoids any possibility of unwanted output being written.
This cannot be used along with --print or --revert. Either will exit

early with an error.
This change is based on https://reviews.reviewboard.org/r/11676/ by

André Klitzing, and was updated to remove some legacy handling of diff

content Unicode strings and to rename the option from --file to

--write.

Tested --print, --write, and --print > filename on Python 2 and 3,

with diffs containing Unicode content. In all cases, the file content

was written/outputted correctly and consistently.
Tested that neither --write nor --print could be used with each other

or with --revert.