fix mypy_primer_comment workflow for more shards (#6533)

.github/workflows/mypy_primer_comment.yml

@@ -38,6 +38,9 @@ jobs:
             fs.writeFileSync("diff.zip", Buffer.from(download.data));
 
       - run: unzip diff.zip
+      # 30000 bytes is about 300 lines, posting comment fails if too long
+      - run: |
+          cat diff_*.txt | head -c 30000 | tee fulldiff.txt
 
       - name: Post comment
         id: post-comment
@@ -46,15 +49,7 @@ jobs:
           github-token: ${{secrets.GITHUB_TOKEN}}
           script: |
             const fs = require('fs')
-            const data = (
-              ['diff_0.txt', 'diff_1.txt']
-              .map(fileName => fs.readFileSync(fileName, { encoding: 'utf8' }))
-              .join('')
-              .substr(0, 30000)  // About 300 lines
-            )
-
-            console.log("Diff from mypy_primer:")
-            console.log(data)
+            const data = fs.readFileSync('fulldiff.txt', { encoding: 'utf8' })
 
             let body
             if (data.trim()) {