R1:4270649abe1d

Increase the size of the Diffusion commit cache

Summary:
Ref T12296. This cache is used to cache Git ref heads (branches, tags, etc). Reasonable repositories may have more than 2048 of these.

When we miss the cache, we need to single-get refs to check them, which is relatively expensive.

Increasing the size of the cache to 65535 should only require about 7.5MB of RAM.

Additionally, fill only as much of the cache as actually fits. The FIFO nature of the cache can get us into trouble otherwise.

If we…