fix(checkpatch): do not check merge commits

Add the --no-merges option when listing patches to check with rev-list
command, when running make checkpatch.

Change-Id: I47f3f5dfe358ed2b960a754f70aec0dc3c2b4536
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/Makefile b/Makefile
index e5ab324..2a1d4d8 100644
--- a/Makefile
+++ b/Makefile
@@ -1318,7 +1318,8 @@
 		echo "    with ${CHECKPATCH_OPTS} option(s)";		\
 	fi
 	${Q}COMMON_COMMIT=$$(git merge-base HEAD ${BASE_COMMIT});	\
-	for commit in `git rev-list $$COMMON_COMMIT..HEAD`; do		\
+	for commit in `git rev-list --no-merges $$COMMON_COMMIT..HEAD`;	\
+	do								\
 		printf "\n[*] Checking style of '$$commit'\n\n";	\
 		git log --format=email "$$commit~..$$commit"		\
 			-- ${CHECK_PATHS} |				\