Truncate Long Report

[shawntabrizi]

Github message body has a maximum of 65536:

(node:1265884) UnhandledPromiseRejectionWarning: HttpError: Validation Failed: {"resource":"IssueComment","code":"custom","field":"body","message":"body is too long (maximum is 65536 characters)"}
    at /home/benchbot/bench-bot/node_modules/@octokit/request/dist-node/index.js:68:23
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async sendRequestWithRetries (/home/benchbot/bench-bot/node_modules/@octokit/auth-app/dist-node/index.js:450:12)
    at async Job.doExecute (/home/benchbot/bench-bot/node_modules/bottleneck/light.js:405:18)
(node:1265884) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 3)
(node:1265884) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.