Connect pre-existing try build perf runs and final merge perf runs for triage

One of the current hassles during perf triage is following up based on the try runs and their results when doing triage of the final merge. It should be relatively easy for us to generate a listing of the try build runs (at least their comparison pages) -- maybe even just "all runs done for the second commits PR" or something like that.

In the future, we may wish to connect perf justification to this with a rust-timer invocation, such that we can at least link to those comments. I'm hesitant to inline them directly into the database, since then there's edits etc, but links should be easy enough.

No files, tests, or entry points are named. Start by tracing how try-build runs and final merge perf runs are represented and surfaced during triage, then determine how a listing of related try runs or comparison pages should be exposed. Done means triage can follow the relevant try-run results from a final merge run without manually searching; future rust-timer links are optional context.