GitDiffParser (and subclasses) now extract UNIX file modes from the

mode lines and symlink targets from the diff data, setting them on the

ParsedDiffFile, eventually making its way into FileDiff.
In the future, we'll be able to show these mode changes in the diff

viewer, or replicate them when applying a diff (technically they already

are, but we'll have the metadata surrounding those changes).

Unit tests pass.