Rescue NoRepository exceptions when fetching patch_id

What does this MR do and why?

This error is returned when either old_revision or new_revision do not exist in the repository. Similar to Gitlab::Git::CommandError, we want to return nil, as it is generally a non-fatal exception, so we should exit gracefully.

Additionally, we're adding logging of both exceptions, so that we can track the incident rate, and will help with debugging customer issues.

Related to #424542 (closed) and !131407 (merged)