Now that ParsedDiffFile expects a ParsedDiffChange argument, it's

time to update our diff parsers to provide.
The base DiffParser handles this for most of our diff parsers. We only

need to be careful to discard the parser if skipping the file. This

requires a little bit of code reorganization, but not much.
GitDiffParser also creates one, but this change is a bit easier. We

simply need to make sure we don't bail immediately after creating one,

if we don't have enough expected lines remaining in the diff file. While

here, I've taken the opportunity to clean up some comments and add

docstrings.
There is an important change in usage of ParsedDiffChange now. If a

diff parser constructs its own instances (which most won't need to do),

it can no longer just throw it away. It'll still be in the list of

files. It must now call discard() to get rid of it. This isn't an

issue in Review Board or Power Pack, and is unlikely to impact anyone in

production, given how uncommon this is.

Unit tests pass.