catfile: Implement `--batch-command` mode behind a FF
Currently, we have two separate subprocess which for git-cat-file(1). One for reading the info and the other for reading the contents. With the introduction of --batch-command
flag in git v2.37.0. We can swap them out for a single process.
We renamed existing code in !5068 (merged) and here we add the new implementation behind a feature flag in this MR. After the rollout of the feature flag, we shall remove the old code and cleanup.