This introduces rbtools.diffs.writers.UnifiedDiffWriter, which is a

io.BytesIO that allows for consistent, faster building of diffs.
Most of our SCMs have historically built diffs as lines that are then

joined together. While that worked, it wasn't as optimal as a BytesIO

should be, and it left so much of the diff building up to each

individual SCMClient, not all of which were consistent in how they built

diffs.
UnifiedDiffWriter provides a nicer interface with more consistent

results. There's functionality for writing original/modified headers,

hunks, and arbitrary lines. These can also be generated straight from a

DiffFileResult, minimizing the work required by SCMClients.
A writer can take a newline string, and an encoding for any Unicode

strings These will probably never need to be changed (and in fact the

encoding shouldn't be, as encodings in diffs is complicated and not

something that should generally be touched at this level). However, we

have these set so that we can have stable values to refer to during diff

building.
Upcoming changes to SCMClients will switch over to diff tools and the

writer for building diffs.

Unit tests passed.
Made use of this in some upcoming changes successfully.