Retitling to clarify that we will use buildset and Gitiles change to identify associated builders to render on the Milo build page.

please update the bug description too. Is this for fast navigation from one pre-submit build of a given CL set to any other pre-submit build of the same CL set?

Updated.

Yes, and for post-submit and release builds in the same buildset. Is Browser CQ setting buildset on pre-submit builds? If so, using buildset alone would be sufficient; we don't need Gerrit CL.

yes, Browser CQ sets buildset on pre-submit builds.

Note that "Gerrit CL" of a build is currently implemented via buildset. The buildset string specifies the CL. Today Browser CQ sets the buildset string directly. It will be switching to v2 API where it does not directly set buildset (but sets input gerrit_changes instead), and buildbucket server will append buildset string in the same format. So anyway, before and after the switch, pre-submit builds will have the buildset. FWIW, pretty much the same applies to post-submit builds and LUCI Scheduler, except s/gerrit cl/gitiles commit/.

With buildset remaining a generic string that defines a set of a build and works for both CL and commit, and given that search by buildset is optimized, it is reasonable to display links to other builds in the same buildsets as the current build (note that one build can have multiple buildsets).