Add pre-commit dprint hook, fix post-checkout hook (microsoft#55799)

scripts/hooks/post-checkout

@@ -1,2 +1,2 @@
 #!/bin/sh
-npm run npx hereby -- generate-diagnostics
+./node_modules/.bin/hereby generate-diagnostics || true

scripts/hooks/pre-commit

@@ -0,0 +1,4 @@
+#!/bin/sh
+if git diff --name-only --cached --relative | xargs --no-run-if-empty ./node_modules/.bin/dprint fmt; then
+    git diff --name-only --cached --relative | xargs --no-run-if-empty git add
+fi

scripts/link-hooks.mjs

@@ -11,6 +11,7 @@ const __dirname = path.dirname(__filename);
 
 const hooks = [
     "post-checkout",
+    "pre-commit",
 ];
 
 hooks.forEach(hook => {