PR bot codegen diff is broken for large diffs

[jdisanti]

Discovered in #1216, it seems like diff2html omits files from the diff when the diff is exceedingly large.

I did the following diagnosis work:

• Modified my local codegen-diff-revisions.py script to not delete the temporary branches (here).
• Ran the script for the changes in the PR linked above.
• Confirmed S3 and several other services had changes in the git diff between the two temporary branches.
• Confirmed the diff2html output did not include these files.

This issue tracks fixing the HTML output. We might need to build a tool or update an existing tool to generate a paginated HTML diff since SDK codegen diffs are always going to be huge.