fix: only promisify raw natives so re-evaluation does not stack wrappers

[erunion]

Problem

The native binding is a process-wide singleton, but the generated lib/nodegit.js assigns promisified wrappers onto the shared native classes (_Tree.lookup = promisify(_Tree.lookup), etc.). Anything that evaluates the package's JS more than once in the same process — notably Jest, which gives every test file a fresh module registry inside a long-lived shard worker — stacks another wrapper layer per evaluation. On Node 22, every call through a util.promisify layer whose inner function already returns a Promise emits:

(node:4686) [DEP0174] DeprecationWarning: Calling promisify on a function that returns a Promise is likely a mistake.

In a downstream CI run this produced thousands of warnings per shard (in escalating bursts as workers accumulated layers), drowning out real test failures. Each extra layer also leaks a pending promise per call.

Why a simple "already wrapped" flag isn't enough

Tagging promisify's output and skipping flagged functions does not fix this (verified against a local reproduction — all 156 warnings remained). After the template promisifies a method, the manual extensions replace it with their own plain promise-returning function (e.g. Commit.lookup = lookupWrapper(Commit) in lib/commit.js). The next evaluation promisifies that unflagged extension wrapper — which is why the --trace-deprecation stacks point at lib/utils/lookup_wrapper.js:27.

Fix

Guard the generated promisify helper so it only ever wraps the raw callback-style natives, which stringify as [native code]. Anything else (a promisify wrapper or an extension wrapper installed by a prior evaluation) is already promise-based and is returned untouched:

var promisify = fn => {
  if (typeof fn !== "function" ||
      !Function.prototype.toString.call(fn).includes("[native code]")) {
    return fn;
  }
  return util.promisify(fn);
};

Regression test

test/tests/reload.js runs test/utils/reload_fixture.js in a child process (so the suite's own module cache is untouched). The fixture evaluates lib/nodegit.js twice in one process — fresh JS module registry, shared native binding, exactly the Jest scenario — exercises Repository.open/Commit.lookup including the lookupWrapper path, and fails on any DEP0174 warning. It exits 1 against the old promisify and 0 with this fix.

Verification

• Double-load reproduction: 156 DEP0174 warnings before → 0 after, with lookups across three evaluations still resolving correctly.
• npm run lint passes; regenerated lib/nodegit.js matches the template output.
• Full mocha suite locally: 418 passing, 9 failing — all 9 are pre-existing environmental failures (network/credential-dependent clone/fetch tests, missing test/id_rsa keys, and init.defaultBranch=main vs tests expecting master); none related to this change.

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