Don't show diffs when listing commits

Normally this script should only list merge commits without conflicts,
so there shouldn't be a diff. But don't pollute the output if there is
a diff.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Author: gilles-peskine-arm

tools/bin/mbedtls-check-missing-changelog

@@ -30,5 +30,5 @@ for c in $merge_commits; do
   if git diff --name-only $c~1 $c | grep -q ChangeLog; then
     continue
   fi
-  git show --oneline $c | grep .
+  git show -s --oneline $c | grep .
 done