Tooltip should default to commit author not committer

When tooltips for the commit data are rendered, they give the name of the user who committed the commit.

https://github.com/benchmark-action/github-action-benchmark/blob/fe4e90e7735310d03ca4b1d8f0eb1d425c55be00/src/default_index_html.ts#L217

This is not always useful, as merge and squash commits are committed by [`web-flow`](https://github.com/web-flow). The author is a better value to use.

I've used the template index to write my own UI, so it's easy to change with custom code, but I think it makes sense to render the default built-in pages that way too.

```diff
- return '\n' + data.commit.message + '\n\n' + data.commit.timestamp + ' committed by @' + data.commit.committer.username + '\n';
+ return '\n' + data.commit.message + '\n\n' + data.commit.timestamp + ' committed by @' + data.commit.author.username + '\n';
```