search: avoid the need to compute repoBranches
Created by: keegancsmith
We don't need to allocate repoBranches anymore. Previously we allocated it since we sent it down as part of the zoekt query. We switched recently to a more efficient encoding. We still used repoBranches for mapping zoekt results back into reporevs. However, we can instead compute that information using the same algorithm we had to compute repoBranches.
Depends on https://github.com/sourcegraph/sourcegraph/pull/25675