Ignore error when retrieving changed PR review files (#21487) (#21524)

Ignore error when retrieving changed PR review files (#21487) (#21524)

When a PR reviewer reviewed a file on a commit that was later gc'ed,
they would always get a `500` response from then on when loading the PR.
This PR simply ignores that error and instead marks all files as
unchanged.
This approach was chosen as the only feasible option without diving into
**a lot** of error handling.

Fixes #21392
Backport of #21487

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>