This PR migrates the calls to git ls-tree to gRPC.

Since ReadDir and Stat used the same git command under the hood, I migrated those two together.

Stat and ReadDir are now simply moved to the backend with the usual layering we did so far.

ListDirectoryChildren effectively was a ReadDir with some post-request processing into a map, so I moved that logic to the only caller instead and made it just use ReadDir.

LsFiles uses the ls-files command which is meant for worktrees and is not a plumbing command so I tried to best replicate its use-cases using ReadDir instead.
There were three use-cases for LsFiles so far:

1. Exposing all files in the repo from GQL for the fuzzy finder (this is a terrible API as it'll create potentially humongous responses but that's not a fault of our API), and I made it use ReadDir with recurse: true instead and I filter out dir entries.
2. The own indexer used it to go over all the files, I made that use ReadDir instead as well.
3. In inference, it was used to get files matching certain patterns. Git ls-tree does not support globbing while ls-files does, so we can not do that anymore. BUT: the logic already did filtering in-app, it just used that to reduce the result set to iterate over. So we decided that for now we will go with a ReadDir and then later we can try to find a better replacement here.

Closes https://github.com/sourcegraph/sourcegraph/issues/47420

Closes https://github.com/sourcegraph/sourcegraph/issues/60413

Closes https://github.com/sourcegraph/sourcegraph/issues/60414

Test plan

Verified that inference still returns the same results. Also browsed files, opened file trees etc from the web app. Existing E2E test suites also still pass.