Historically, our diff parsing infrastructure was built around a base

set of manual line-by-line parsing rules, which could be specialized.

The result would be a list of files. This was expanded a bit in 4.0 and

subsequent releases to collect and handle additional data (such as being

able to populate FileDiff.extra_data), but the core design remained.
With DiffX coming, we need more. This change expands our diff parsing

support.
New ParsedDiff and ParsedDiffChange classes were added, which

represent information on a diff as a whole, and a change within it,

respectively. These both provide support for populating extra_data,

which will be set in DiffSet and DiffCommit when parsing a diff.
A new BaseDiffParser has been introduced as the new base class, which

DiffParser subclasses. The new base class does very little, and is

more of an interface. It contains no built-in parsing logic. This will

allow for more specialized parsers with their own logic.
BaseDiffParser and DiffParser now provide a parse_diff() method,

which is the new method to call to parse a diff. This replaces

parse(), which still exists in DiffParser.
For new subclasses of BaseDiffParser, a parse_diff() implementation

will be responsible for parsing into a series of ParsedDiff,

ParsedDiffChange, and ParsedDiffFile objects.
For subclasses of DiffParser, parse() can still be overridden. The

new default DiffParser.parse_diff() will set up the new objects as

parsed_diff and parsed_diff_change attributes.
This new model will make it easier to implement DiffX, and will give us

some good capabilities down the road. For the moment, there are still

limitations based on our existing process of uploading and parsing

diffs. The main one being that, while a ParsedDiff (and later a DiffX)

can in theory have multiple changes, it's not actually allowed through

our parsing. Clients of the API will need to provide diffs with only a

single change, until we're able to implement the appropriate changes

in the diff upload API and related code (probably post-4.0 sometime).

Unit tests pass on Python 2 and 3.
Tested posting diffs for review, without any issues.