gitea: a merge poll looks only at repositories that moved, one pass at a time

With the poll the only announcer of a merge, its cost showed: every 30 s it
asked every repository for its pull requests, a pass outlasted the tick, and
passes piled up beside each other — a merge was announced four and a half
minutes late, and two passes at once could each announce it. A pass now asks
only repositories updated since a minute before the last look, and the next
pass starts when this one ends. hq issue 250.

modules/gitea/client.ts

@@ -17,6 +17,8 @@ export interface GiteaRepo {
   description?: string;
   html_url: string;
   default_branch?: string;
+  /** When anything last moved in it — a push, and so a merge. */
+  updated_at?: string;
 }
 
 /** An issue, with its labels flattened to names. */
@@ -356,6 +358,7 @@ export class GiteaClient {
       description: r.description || undefined,
       html_url: r.html_url,
       default_branch: r.default_branch,
+      updated_at: r.updated_at ?? undefined,
     };
   }
 
@@ -598,3 +601,11 @@ export class GiteaAdmin {
     GiteaAdmin.fail(`/admin/users/${username}`, res);
   }
 }
+
+/** The repositories that moved at or after a moment: every one when there is no moment yet, and one whose
+ *  update time is not known, so a forge that does not say is asked as before (novox/hq issue 250). */
+export function movedSince(repos: GiteaRepo[], floor: string): GiteaRepo[] {
+  if (!floor) return repos;
+  const at = Date.parse(floor);
+  return repos.filter((r) => !r.updated_at || !(Date.parse(r.updated_at) < at));
+}