Fix diffing of rendered text file attachments on Python 3.
Review Request #1752 — Created Oct. 10, 2019 and submitted
Rendered text file attachments (such as Markdown) are rendered as Unicode, for output to the client. These were getting passed directly to the differ, which itself takes byte strings. On Python 2, this all worked out fine. On Python 3, this blew up pretty early, but in a way that was hard to diagnose due any errors coming from subclasses just being logged (with a massively-huge log message) and then the exception ignored. The Markdown review UI specifically had another issue where we were passing in a file handle containing byte strings, which was then getting written back as byte strings but to a `StringIO` expecting Unicode strings. While the core issue lives in Djblets (and there's another patch up that aims to fix this), it's better if we just read the file content and convert to Unicode first, since the Markdown library doesn't stream the file pointer anyway (it just grabs all the data at once). This change most of these issues by doing the following: 1) Ensures we're always passing the right string types around (both to Markdown and the diff chunk generators). 2) Switches the Markdown rendering call to `render_markdown()`, passing it a Unicode string instead of a file handle. 3) Adds new unit tests covering all this. It does not address the overzealous exception sandboxing. That will be taken care of separately. Testing Done: Unit tests passed. Verified I could view Markdown files in both rendered and source mode, with diffs. Reviewed at https://reviews.reviewboard.org/r/10709/