fix(tests): scrub GIT_* env vars leaking from the pre-commit hook into git-fixture tests

server/lib/update-check.js's execGit() and two test helpers
(lanes-cli.test.js, update-check.test.js) shelled out to git with an
explicit `cwd` but no `env` override. A parent git hook process (this
repo's own .husky/pre-commit, which runs `npm run test:server`) sets
GIT_DIR/GIT_INDEX_FILE in its own environment; those leak to every
child process and take precedence over `cwd` for repo discovery, so
every git command these tests ran against their throwaway tmp repos
was silently redirected at the real repo running the hook instead —
reproduced firsthand as four foreign "init"/"fixture" commits
overwriting a worktree branch mid pre-commit run. Fixes it the same
way server/lib/worktree.js already documented and did for its own git
calls: strip the GIT_* vars before exec.

server/__tests__/lanes-cli.test.js

@@ -45,9 +45,25 @@ const CLI = path.join(__dirname, "..", "..", "bin", "ccam.js");
 let server;
 let BASE;
 
+// Strip GIT_* vars a parent git hook (e.g. the pre-commit hook running this
+// very suite) sets in its own environment — those leak to every child
+// process and override an explicit `cwd`, so without this a git command
+// meant for this test's throwaway tmp repo silently operates on the real
+// repo running the hook instead.
+const GIT_ENV = { ...process.env };
+delete GIT_ENV.GIT_DIR;
+delete GIT_ENV.GIT_WORK_TREE;
+delete GIT_ENV.GIT_INDEX_FILE;
+delete GIT_ENV.GIT_COMMON_DIR;
+delete GIT_ENV.GIT_OBJECT_DIRECTORY;
+delete GIT_ENV.GIT_ALTERNATE_OBJECT_DIRECTORIES;
+delete GIT_ENV.GIT_PREFIX;
+delete GIT_ENV.GIT_NAMESPACE;
+delete GIT_ENV.GIT_CONFIG_PARAMETERS;
+
 function git(args, cwd) {
   return new Promise((resolve, reject) => {
-    const child = spawn("git", args, { cwd });
+    const child = spawn("git", args, { cwd, env: GIT_ENV });
     let stderr = "";
     child.stderr.on("data", (chunk) => (stderr += chunk));
     child.on("error", reject);