tools: ignore merge commits in signoff check

Signed-off-by: Pablo de Lara <[email protected]>

tools/check_format.sh

@@ -77,7 +77,7 @@ done <<< $(git ls-files -s '*.sh')
 
 
 echo "Checking for signoff in commit message..."
-if ! git log -n 1 --format=%B | grep -q "^Signed-off-by:" ; then
+if ! git log -n 1 --format=%B --no-merges  | grep -q "^Signed-off-by:" ; then
     hash=`git rev-parse --short HEAD`
     echo "  Commit $hash not signed off. Please read src/CONTRIBUTING.md"
     rc=1