truncate PR body to comply with github api limits

If you make a large number of commits, then the PR body can be generated to exceed 65,536 characters, which will cause the PR to fail.

```js
error: Uncaught (in promise) HttpError: Validation Failed: {"resource":"Issue","code":"custom","field":"body","message":"body is too long (maximum is 65536 characters)"} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request
at file:///home/runner/.cache/deno/npm/registry.npmjs.org/@octokit/request/8.4.0/dist-node/index.js:125:21
at eventLoopTick (ext:core/01_core.js:175:7)
at async requestWithGraphqlErrorHandling (file:///home/runner/.cache/deno/npm/registry.npmjs.org/@octokit/plugin-retry/6.0.1/dist-node/index.js:71:20)
at async Job.doExecute (file:///home/runner/.cache/deno/npm/registry.npmjs.org/bottleneck/2.19.5/light.js:405:18)
Error: Process completed with exit code 1.
```