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/adoption.test.ts

@@ -55,6 +55,7 @@ import { catalogueRoot } from "../../src/repos.ts";
 import { ready, returnTo, keep } from "../../src/warm.ts";
 import { labIsUsable, destroyAll, foundationBundle, catalogueModule, catalogueManifest } from "./harness.ts";
 import { genesis, type GenesisOptions } from "./genesis.ts";
+import { carry } from "./forge.ts";
 
 const SCENARIO = "adoption";
 const CONTROL = "anchor";
@@ -246,11 +247,7 @@ async function raiseForge(): Promise<Record<string, string>> {
   const heads: Record<string, string> = {};
   try {
     for (const [name, checkout] of Object.entries(REPOS)) {
-      const bare = join(dir, `${name}.git`);
-      execFileSync("git", ["init", "-q", "--bare", bare]);
-      execFileSync("git", ["-C", checkout, "push", "-q", "--force", bare,
-        "HEAD:refs/heads/main", "HEAD:refs/heads/lab"], { stdio: "pipe" });
-      heads[name] = execFileSync("git", ["-C", checkout, "rev-parse", "HEAD"], { encoding: "utf8" }).trim();
+      heads[name] = carry(checkout, join(dir, `${name}.git`), ["main", "lab"]);
     }
     const tar = join(tmpdir(), `mesh-lab-forge-${process.pid}.tar`);
     execFileSync("tar", ["-cf", tar, "-C", dir, "."]);