Make commit cache in git plugin configurable

[j-lanson]

Right now, the plugins/git plugin has an internal Mutex<(.., Vec<RawCommit>) that acts as a cache of RawCommits for the last-queried repo. This is helpful for performance, since we usually will run multiple analyses that run on a single repo and if we can skip re-calcing the RawCommits, it's much faster.

However, right now it's hard-coded to only work for 1 repo. Update the cache size to be configurable through the get_config() gRPC call, and make it default to commit_cache_size = 1.

This will require a version bump to the git plugin.