codeintel: Batch commit exists checks in find refs operations
Created by: efritz
Continuing from #32908. This PR attempts to batch the following waterfall spans:
Since we can't batch all of these operations, we batch the code-intel-team side of things, and press the remaining loop as far to the gitserver client invocation as possible. We'll open up a request to enable batch requests in gitserver, but that may be a while off. An intermediate step may be parallelization of these calls, but we should measure the impact of this PR as-is first.
Test plan
Updated existing unit tests. I expect the existing integration tests to show the same behavior externally.