Historically, Review Board has depended on source files to either us a

UTF-8 or ISO-8859-15 encoding, or for the repository to supply a list of

compatible encodings. If a special encoding were ever used for a file,

and the repository later had its encodings list changed to exclude it,

file would no longer be able to be viewed in the diff viewer.
This encoding list was computed up-front when generating a diff for

file. We'd fetch the repository encoding list, pass it to calls like

get_original_file(), which would then pass that on to

convert_to_unicode(), which would try the built-in encodings and the

provided list of encodings. The caller was always responsible for

fetching or computing that encoding list up-front before calling any of

these methods.
This change rewrites all of this to make the process more dependent on

the FileDiff we're working on, and to record a known working encoding

for future use.
FileDiff.extra_data can now contain an 'encoding' key (conveniently

exposed as FileDiff.encoding), which specifies an explicit encoding to

use. This could in theory be set during diff parsing (which requires

some additional work still), but it will be set after we've successfully

converted a source file to Unicode and back.
If not set, we still respect the provided encoding list (though this is

now deprecated in get_original_file()), and then look up from the

repository. This will allow us to later work with FileDiffs backed by

multiple repositories without placing unnecessary burdens on the caller.
The machinery for diff chunk generation has been updated as well. Now,

RawDiffChunkGenerator.generate_chunks() can be given an encoding list

for the old and new content. This defaults to the encoding list normally

provided to the chunk generator, but subclasses (DiffChunkGenerator)

can pass in separate encoding lists, allowing for two FileDiffs on an

interdiff to be in separate encodings without breaking chunk generation.
Note that it's still possible for a patch to contain content in another

encoding that doesn't match what's in FileDiff.encoding. The chunk

generator will still try to do the right thing in this case, falling

back to the repository encodings.

Unit tests passed.
Made sure my various test diffs and interdiffs were still working as

expected.