Mute a bit codecov report. (#643)

Codecov report are a bit noisy, this mute them a bit y removing the
large graph and postigin only if there is a coverage change.

Author: Carreau

codecov.yml

@@ -7,3 +7,9 @@ coverage:
     patch:
       default:
         target: 100
+comment:
+  layout: "diff, files"
+  behavior: default
+  require_changes: true  # if true: only post the comment if coverage changes
+  branches:               # branch names that can post comment
+    - "master"