use swift-cmark/gfm for testing main

QuietMisdreavus · QuietMisdreavus · commit e358502af3a5 · 2022-05-03T11:27:46.000-06:00
diff --git a/common.py b/common.py
@@ -34,7 +34,7 @@
     'main': {
         'llvm-project': 'stable/20211026',
         'swift': 'main',
-        'cmark': 'main',
+        'cmark': 'gfm',
         'ninja': 'release',
         'llbuild': 'main',
         'swiftpm': 'main',
