Unbreak installing from Git

`save-git-history.ts` breaks installing BCD from Git (in a fit of irony,
there won't be a `.git` if you use `npm install
github:mdn/browser-compat-data`).

While I'm here, I've stopped the script from running if you, like me,
have `HUSKY=0` set.

package.json

@@ -92,7 +92,8 @@
     "yargs": "~17.7.0"
   },
   "scripts": {
-    "postinstall": "node --loader=ts-node/esm --no-warnings=ExperimentalWarning scripts/save-git-history.ts",
+    "postinstall": "(test HUSKY = 0 || (husky || true)) && echo 'not in worktree, skipping save-git-history') || npm run save-git-history",
+    "save-git-history": "node --loader=ts-node/esm --no-warnings=ExperimentalWarning scripts/save-git-history.ts",
     "prepare": "(husky || true) && npm run gentypes && npm run build",
     "diff": "node --loader=ts-node/esm --no-warnings=ExperimentalWarning scripts/diff.ts",
     "unittest": "NODE_ENV=test c8 mocha index.test.ts --recursive \"{,!(node_modules)/**}/*.test.ts\"",

scripts/save-git-history.ts

@@ -10,6 +10,10 @@ import { getBranchName, getHashOfHEAD } from './lib/git.js';
 const HUSKY_ROOT = '.husky/_/';
 const HISTORY_FILE = HUSKY_ROOT + 'history';
 
-if ('main' === getBranchName() && fs.existsSync(HUSKY_ROOT)) {
+if (
+  'main' === getBranchName() &&
+  fs.existsSync(HUSKY_ROOT) &&
+  process.env['HUSKY'] !== '0'
+) {
   fs.writeFileSync(HISTORY_FILE, getHashOfHEAD());
 }