ParsedDiffFile and FileDiff both contained information on whether a

file was a symlink, but nothing more than that. There was no information

on targets, meaning that it was up to the diff implementation and

patch to generate a suitable symlink for display or when applying a

patch. This wasn't ideal.
ParsedDiffFile now contains typed attributes for old and new symlink

targets. These are optional, but can be set by the diff parser. The

DiffX parser is currently the only one that sets these.
FileDiff provides attributes for getting or setting all symlink state.

These are is_symlink, old_symlink_target, and new_symlink_target.

These all wrap extra_data, giving us a formal way of working with

these attributes.
An upcoming SCM will need thess. A separate upcoming change will enable

this support for Git (and Mercurial by extension). Eventually, we'll be

able to use these to avoid some hacks we're currently using to render

symlink changes in diffs.
This will also be useful for Review Bot and RBTools, which will benefit

from being able to know how to correctly manage symlinks.

Unit tests pass.