Let a bed's forge take the lab's shallow checkouts (issue 689)

The lab module clones every repository with --depth 50, and a fresh bare
repository refuses a push whose history ends in a cut, so the genesis bed
raised its anchor and forge and then skipped. The three beds that carry a
checkout into a forge now share one helper that sets receive.shallowUpdate
on the bare repository. Unshallowing instead cannot work in a lab run: the
forge credential is removed once the run's clones are done.

test/integration/replay92.test.ts

@@ -43,6 +43,7 @@ import { destroy, exec, push } from "../../src/lifecycle/operate.ts";
 import { bootstrapBinaryPath, hostBinaryPath } from "../../src/lifecycle/place.ts";
 import { labIsUsable, destroyAll, foundationBundle } from "./harness.ts";
 import { genesis, type GenesisResult } from "./genesis.ts";
+import { carry } from "./forge.ts";
 
 const SCENARIO = "genesis-untrusted";
 const ANCHOR = "anchor";
@@ -98,13 +99,9 @@ async function must(machine: string, command: string, timeoutMs?: number): Promi
 /** The control plane's repository at the checkout's HEAD, served from the forge machine over git's protocol. */
 async function raiseForge(): Promise<string> {
   const dir = mkdtempSync(join(tmpdir(), "mesh-lab-r92-forge-"));
-  const head = execFileSync("git", ["-C", controller, "rev-parse", "HEAD"], { encoding: "utf8" }).trim();
+  let head: string;
   try {
-    const bare = join(dir, "mesh-controller.git");
-    execFileSync("git", ["init", "-q", "--bare", bare]);
-    // No collection after the push: one run in the background rewrites the objects while they are carried.
-    execFileSync("git", ["-C", bare, "config", "receive.autogc", "false"]);
-    execFileSync("git", ["-C", controller, "push", "-q", "--force", bare, "HEAD:refs/heads/main"], { stdio: "pipe" });
+    head = carry(controller, join(dir, "mesh-controller.git"));
     const tar = join(tmpdir(), `mesh-lab-r92-forge-${process.pid}.tar`);
     execFileSync("tar", ["-cf", tar, "-C", dir, "."]);
     await push(instanceId, FORGE, tar, "/tmp/forge.tar");