Diff parsing was intended to work with byte strings, and generally did a

pretty good job of this. Still, we had no proper enforcement, and we had

some places where byte strings were being compared to Unicode strings,

or temporarily transformed into Unicode strings. Much, though not all,

of the type inconsistency occurred in the parse_diff_revision()

methods on SCMTool subclasses. The rest was somewhat harmless, setting

empty strings or default revision identifiers in ParsedDiffFile

attributes, which then just made their way ultimately into other

function calls or into FileDiff attributes.
Python 2's ability to interchange byte strings and Unicode strings meant

that this all pretty well worked, even when wrong. That changes with

Python 3. Byte strings and Unicode strings are now very different, and

we need to ensure we're working only with byte strings for all parsing.

We also need to make sure we have somewhat of a migration path for the

parsing that's capable of informing when things are wrong.
This change goes through our diff parsing and ensures that they're

exclusively working with byte strings, updating string literals and

using io.BytesIO instead of StringIO (which is Unicode on Python 3).

All parse_diff_revision() methods now assert the types coming in, and

the code that calls it now asserts the types on the results.

DiffParser also now requires a byte string, rather than accepting a

Unicode string, and asserts on this.
The other big part of this change is the data going into and coming out

of ParsedDiffFile. This class is very old, and we had old attributes

with old-style names that accepted any value with the assumption that

it'd all get normalized to bytes at some point. Now, we have newer

attributes that replace the old ones, but enforce type safety. The old

attribute names continue to work and will cast to the right type, but

will also emit deprecation warnings, helping us or third-parties to

catch any issues.
The diff viewer code all uses the new attributes, but there's still a

bunch of SCMTool code and tests that use the old attributes, causing

warnings to appear. Those will be tackled separately.

Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).
Tested posting changes for review.