Add "internal.commit.search" to replace the cache bypass mode of "diffusion.querycommits"

Summary:
Ref T13552. Commit parsers currently invoke a special mode of "diffusion.querycommits", which is an older frozen method.

The replacement, "diffusion.commit.search", is not really appropriate for low-level access. This mode of having a single method which operates in "cache" or "non-cache" modes also ends up in a lot of unnecessary field shuffling.

Provide "internal.commit.search" as a modern equivalent that returns a "DiffusionCommitRef"-compatible structure.

Test Plan: Executed "internal.commit.search", got sensible low-level commit results.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21443