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 insert "A, B, C, D" and then lookup A, B, C, D, but the cache has maximum size 3, we get this:

- Insert A, B, C, D: cache is now "B, C, D".
- Lookup A: miss, single get, insert, purge, cache is now "C, D, A".
- Lookup B: miss, singel get, insert, purge, cache is now "D, A, B".

Test Plan:
- Reduced cache size to 5, observed reasonable behavior on the `array_slice()` locally with `bin/repository update` + `var_dump()`.
- Used this script to estimate the size of 65535 cache entries as 7.5MB:

```
epriestley@orbital ~ $ cat size.php
<?php

$cache = array();

$mem_start = memory_get_usage();
for ($ii = 0; $ii < 65535; $ii++) {
$cache[sha1($ii)] = true;
}

echo number_format(memory_get_usage() - $mem_start)." bytes\n";
epriestley@orbital ~ $ php -f size.php
7,602,176 bytes
```

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12296

Differential Revision: https://secure.phabricator.com/D17409