This is a cleanup change that improves readability and reduces attribute

access for some diff viewer logic, in preparation for some larger

changes coming soon to that code.
DiffChunkGenerator.get_chunks_uncached() had a large amount of

redundant attribute lookups when calculating state needed to build the

chunks. This was unnecessary, and just made the code a little more

noisy and technically slower than needed. We now pull in a few of these

variables into locals at the start of the method.
We also had a lot of calls to get_original_file(),

get_original_file_from_repo(), get_patched_file(), patch(), and

the PatchError constructor that were using positional arguments. Some

of these function signatures are going to get some changes soon, so to

help prepare for that and to make the code more self-documenting, this

change also updates these calls to pass arguments as keywords.
This uncovered some incorrect (but otherwise harmless) calls in some of

our unit tests for patching that would have been avoided if using

keyword arguments to begin with.

Unit tests pass.
Tested that diffs and interdiffs work correctly.