Make markdown the only output option for the completion metrics tool

Change-Id: Id4c2020ea85453980f1ed044552b76f13316c7b1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189460
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

https://dart.googlesource.com/sdk/+/8f2ca0b2482bf6acde1e90aab3745de407d4e51a
diff --git a/commits.json b/commits.json
index 2f7188e..6e69ae2 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"09b2468785a3e07132cac1fba99b1beafdf9dcec",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
-  "sdk":"637d4f84076faecbbf87c03e943a1bdce2ebe93f"
+  "sdk":"8f2ca0b2482bf6acde1e90aab3745de407d4e51a"
 }
\ No newline at end of file