Ignore GIT_* environment when reading the local build from git

A git hook runs with GIT_DIR set, and with it git treats any directory as
the top of that repository, so gitBuild() reported a commit for
directories outside a checkout and the build test failed under the
pre-push hook. Run git with every GIT_* variable removed so discovery
depends only on the directory, and test the GIT_DIR case.

Author: pcarleton

src/hosted/build.test.ts

@@ -1,4 +1,5 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
+import { execFileSync } from 'node:child_process';
 import { mkdtempSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
@@ -41,4 +42,20 @@ describe('server build', () => {
     expect(gitBuild(__dirname)).toBeUndefined();
     expect(gitBuild(mkdtempSync(join(tmpdir(), 'no-git-')))).toBeUndefined();
   });
+
+  it('ignores GIT_DIR in the environment, as a git hook sets it', () => {
+    const root = packageRoot(__dirname)!;
+    const gitDir = execFileSync('git', ['rev-parse', '--absolute-git-dir'], {
+      cwd: root,
+      encoding: 'utf8'
+    }).trim();
+    vi.stubEnv('GIT_DIR', gitDir);
+    try {
+      expect(gitBuild(root)).toMatch(/^[0-9a-f]{7,40}(-dirty)?$/);
+      expect(gitBuild(__dirname)).toBeUndefined();
+      expect(gitBuild(mkdtempSync(join(tmpdir(), 'no-git-')))).toBeUndefined();
+    } finally {
+      vi.unstubAllEnvs();
+    }
+  });
 });

src/hosted/git-build.ts

@@ -11,9 +11,24 @@ import { dirname, join } from 'node:path';
 
 const PACKAGE_NAME = '@modelcontextprotocol/conformance';
 
+/**
+ * The environment without git's own GIT_* variables. A git hook runs with
+ * GIT_DIR (and others) set, and with GIT_DIR set git treats any directory
+ * as the top of that repository; without them, discovery depends only on
+ * the directory git runs in.
+ */
+function gitEnv(): NodeJS.ProcessEnv {
+  return Object.fromEntries(
+    Object.entries(process.env).filter(
+      ([name]) => !name.toUpperCase().startsWith('GIT_')
+    )
+  );
+}
+
 function git(dir: string, args: string[]): string {
   return execFileSync('git', args, {
     cwd: dir,
+    env: gitEnv(),
     encoding: 'utf8',
     stdio: ['ignore', 'pipe', 'ignore'],
     timeout: 2000