BUILD: make the release script use shortlog for the final changelog

It used to reuse the same command producting the list for the changelog,
requiring to run shortlog manually.
diff --git a/scripts/announce-release b/scripts/announce-release
index 7fc3596..d70c33c 100755
--- a/scripts/announce-release
+++ b/scripts/announce-release
@@ -169,7 +169,7 @@
 
 (echo "---"
  echo "Complete changelog :"
- git log --oneline --reverse --format="  - %s" "v$OLD".."v$NEW^"
+ git shortlog "v$OLD".."v$NEW^"
  echo "---"
 ) >> "$OUTPUT"