cmd/coordinator/internal/legacydash: uncap maintner request

We've been seeing the coordinator fail to schedule subrepo runs on
release branches. As far as I can tell, the problem is the request to
maintner: branch = "mixed" means to get at least one commit from each
branch in the repository, but we're asking for 30 commits when we have
50-some branches. So obviously some are going to be left out. That
absence breaks findWork when it tries to map commits to branches.

I don't fully understand which branches happen to be included and which
don't, but this behavior seems clearly wrong. Let maintner decide how
many responses to send back, currently hardcoded at 500.

For golang/go#48523.

Change-Id: I1792e89de96e83ceed6a430a7456b119f633869b
Reviewed-on: https://go-review.googlesource.com/c/build/+/431355
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cmd/coordinator/internal/legacydash/ui.go

@@ -414,11 +414,15 @@ func dashboardRequest(view dashboardView, r *http.Request) (*apipb.DashboardRequ
 	if branch == "" {
 		branch = "master"
 	}
+	maxCommits := commitsPerPage
+	if branch == "mixed" {
+		maxCommits = 0 // let maintner decide
+	}
 	return &apipb.DashboardRequest{
 		Page:       int32(page),
 		Branch:     branch,
 		Repo:       repo,
-		MaxCommits: commitsPerPage,
+		MaxCommits: int32(maxCommits),
 	}, nil
 }