GitHub leaves patch out of a commit's files entries for binary files

and for diffs that are too large. Historically we silently skip these

types of files and leave them out of the diff. With the move to Pydantic

for parsing GitHub API responses, we didn't give FileEntry.patch a

default, so Pydantic treated it as required. This caused get_change()

to fail with an "Unexpected response from GitHub" for any commit that

contained those files.
Now we default the field to None so that we go back to the old silent

skipping behvaiour. I have an upcoming change that will improve things

to include these files in the diff.

Tested creating a review request from a GitHub commit through the

  UI that contained binary files. It failed before this fix.
Ran unit tests.