R1:ed86c42b26b0

Improve performance of repository discovery in repositories with >65K refs

Summary:
Ref T13593. The commit cache in this Engine has a maximum fixed size (currently 65,535 entries).

If we execute discovery in a repository with more refs than this (e.g., 180K), we get fast lookups for the first 65,535 refs and slow lookups for the remaining refs.

Instead, divide the refs into chunks no larger than the cache size, and perform an explicit cache fill before each chunk is processed.

Test Plan:
- Created a…