Fix prettier precommit script to not error if there are only deleted files (#2641)

Author: hsubox76

tools/gitHooks/prettier.js

@@ -63,7 +63,12 @@ async function doPrettierCommit() {
     console.error(e);
     return process.exit(1);
   }
-  const diff = await git.diff(['--name-only', 'origin/master...HEAD']);
+  const diff = await git.diff([
+    '--name-only',
+    'origin/master...HEAD',
+    '--diff-filter',
+    'd'
+  ]);
   // Only run on .js or .ts files.
   const targetFiles = diff.split('\n').filter(line => line.match(/(js|ts)$/));
   if (targetFiles.length === 0) return;