Removes redundant work from the pull request conversation tab.
- Base and head refs resolve through the `cat-file` batch of the git
repos the callers already hold, instead of two `git rev-parse`
processes, reducing the number of git processes from 9 to 7 per page,
about 17 ms faster locally.
- Sidebar reviewers are batch-loaded and code-review comments reuse
their loaded poster, reducing the number of SQL queries from 387 to 358
on a PR with 169 timeline entries and 15 reviews.
Also fixes the commit list of merged pull requests, which resolved the
merge base as a branch name and returned 500.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>