fix(checkpatch): detect issues in commit message

The current implementation of make checkpatch does not detect issues
when Signed-off-by line is missing. The solution is to concatenate
the output of both git log and git diff before executing checkpatch
script.

Change-Id: Ic2d6f8bc10316d73b72a73a1807aa36cdec6900d
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
diff --git a/Makefile b/Makefile
index baf8431..b440847 100644
--- a/Makefile
+++ b/Makefile
@@ -1634,11 +1634,10 @@
 	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} |				\
-			${CHECKPATCH} ${CHECKPATCH_OPTS} - || true;	\
-		git diff --format=email "$$commit~..$$commit"		\
-			-- ${CHECK_PATHS} |				\
+		( git log --format=email "$$commit~..$$commit"		\
+			-- ${CHECK_PATHS} ;				\
+		  git diff --format=email "$$commit~..$$commit"		\
+			-- ${CHECK_PATHS}; ) |				\
 			${CHECKPATCH}  ${CHECKPATCH_OPTS} - || true;	\
 	done