perf(git): parallelize submodule expansion and skip impossible submodule lookups

- expandSubmodules now fans out with Promise.all over the dirty
  submodules and concatenates results in input order, so the polled
  status endpoint no longer pays serial git status/rev-parse spawns
  (P1).
- submoduleForPath bails out before spawning git config when the path
  contains no '/' or the repo has no .gitmodules, removing a spawn
  from every diff call in plain repos (P2).
- Rename submodulePaths() to configuredSubmodulePaths() and the
  expandSubmodules local to dirtySubmodulePaths to disambiguate the
  two concepts (N2).

src/server/git/gitService.ts

@@ -1,5 +1,6 @@
 import { createHash } from "node:crypto";
 import { spawn } from "node:child_process";
+import { existsSync } from "node:fs";
 import { join } from "node:path";
 import type { GitDiffResponse, GitFileState, GitStatusFile, GitStatusResponse } from "../../shared/apiTypes.js";
 import { normalizeRelativePath } from "../workspaces/pathSafety.js";
@@ -49,12 +50,36 @@ export async function gitStatus(cwd: string): Promise<GitStatusResponse> {
  * unchanged) is intentionally not surfaced as a pointer entry.
  */
 async function expandSubmodules(cwd: string, parsed: ParsedStatus, topRaw: string): Promise<GitStatusResponse> {
-  const files: GitStatusFile[] = [...parsed.files];
-  const submodulePaths: string[] = [];
-  let extraForHash = "";
+  // Fan out concurrently — one `git status` per dirty submodule plus one
+  // `git rev-parse` per unstaged pointer move — then concatenate in input
+  // order so the file list and hash are identical to a serial pass.
+  const expanded = await Promise.all(parsed.submodules.map(async (sub) => ({ path: sub.path, ...(await expandSubmodule(cwd, sub)) })));
 
-  for (const sub of parsed.submodules) {
-    submodulePaths.push(sub.path);
+  const files: GitStatusFile[] = [...parsed.files];
+  const dirtySubmodulePaths: string[] = [];
+  let extraForHash = "";
+  for (const part of expanded) {
+    dirtySubmodulePaths.push(part.path);
+    files.push(...part.files);
+    extraForHash += part.extraForHash;
+  }
+
+  return {
+    isGitRepo: true,
+    hash: hash(topRaw + extraForHash),
+    ...(parsed.branch === undefined ? {} : { branch: parsed.branch }),
+    ...(parsed.upstream === undefined ? {} : { upstream: parsed.upstream }),
+    ...(parsed.ahead === undefined ? {} : { ahead: parsed.ahead }),
+    ...(parsed.behind === undefined ? {} : { behind: parsed.behind }),
+    files,
+    submodules: dirtySubmodulePaths,
+  };
+}
+
+/** Expand one dirty submodule: the pointer entry first, then its inner files. */
+async function expandSubmodule(cwd: string, sub: SubmoduleRecord): Promise<{ files: GitStatusFile[]; extraForHash: string }> {
+  const files: GitStatusFile[] = [];
+  let extraForHash = "";
   if (sub.commitChanged) {
     files.push({
       path: sub.path,
@@ -66,8 +91,8 @@ async function expandSubmodules(cwd: string, parsed: ParsedStatus, topRaw: strin
   }
   if (sub.hasModifiedContent || sub.hasUntrackedContent) {
     const inner = await runGit(join(cwd, sub.path), ["status", "--porcelain=v2", "--untracked-files=all", "-z"]);
-      if (inner.code !== 0) continue; // uninitialized / unreadable submodule: skip silently
-      extraForHash += `\0${sub.path}\0${inner.stdout}`;
+    if (inner.code === 0) {
+      extraForHash = `\0${sub.path}\0${inner.stdout}`;
       const innerFiles = parseStatus(inner.stdout, { deferSubmodules: false }).files;
       for (const file of innerFiles) {
         files.push({
@@ -77,18 +102,9 @@ async function expandSubmodules(cwd: string, parsed: ParsedStatus, topRaw: strin
         });
       }
     }
+    // non-zero exit: uninitialized / unreadable submodule — skip silently
   }
-
-  return {
-    isGitRepo: true,
-    hash: hash(topRaw + extraForHash),
-    ...(parsed.branch === undefined ? {} : { branch: parsed.branch }),
-    ...(parsed.upstream === undefined ? {} : { upstream: parsed.upstream }),
-    ...(parsed.ahead === undefined ? {} : { ahead: parsed.ahead }),
-    ...(parsed.behind === undefined ? {} : { behind: parsed.behind }),
-    files,
-    submodules: submodulePaths,
-  };
+  return { files, extraForHash };
 }
 
 async function resolveSubmoduleToCommit(cwd: string, sub: SubmoduleRecord): Promise<string> {
@@ -154,7 +170,7 @@ async function isUntracked(cwd: string, path: string): Promise<boolean> {
 }
 
 /** Configured direct-submodule paths (depth 1), read from `.gitmodules`. */
-async function submodulePaths(cwd: string): Promise<string[]> {
+async function configuredSubmodulePaths(cwd: string): Promise<string[]> {
   // `-z` emits `<key>\n<value>\0` records; keys may themselves contain spaces
   // (`submodule.my sub.path`), so splitting lines at the first space mangles
   // paths with spaces in them.
@@ -172,7 +188,12 @@ async function submodulePaths(cwd: string): Promise<string[]> {
 
 /** The submodule that strictly contains `path`, if any (longest match wins). */
 async function submoduleForPath(cwd: string, path: string): Promise<string | undefined> {
-  const subs = await submodulePaths(cwd);
+  // Cheap bail-outs before spawning `git config`: a path strictly inside a
+  // submodule always contains `/`, and without `.gitmodules` there are no
+  // configured submodules to look up (every diff call used to pay this spawn).
+  if (!path.includes("/")) return undefined;
+  if (!existsSync(join(cwd, ".gitmodules"))) return undefined;
+  const subs = await configuredSubmodulePaths(cwd);
   let best: string | undefined;
   for (const sub of subs) {
     if (sub !== "" && path.startsWith(`${sub}/`) && (best === undefined || sub.length > best.length)) best = sub;