gitaly: use git.catfile_cache_size=5

Gitaly has a "catfile cache" now, which keeps git cat-file processes around for re-use within the scope of a rails request. These processes use system resources (process table entries, file descriptors) and on desktop systems these can be constrained.

This change makes sure the cache is fairly small in development (5 processes) so we don't risk errors on macOS.

Related to gitaly#1677 (closed)