make checkdiff does CI documentation checks (#900)

Fixes #744

Makefile

@@ -49,6 +49,9 @@ YFLAGS		?= -Wall
 BISON		:= bison
 RM		:= rm -rf
 
+# Used for checking pull requests
+BASE_REF	:= origin/master
+
 # Rules to build the RGBDS binaries
 
 all: rgbasm rgblink rgbfix rgbgfx
@@ -189,9 +192,8 @@ checkcodebase:
 # the first common commit between the HEAD and origin/master.
 # `.y` files aren't checked, unfortunately...
 
-BASE_REF:= origin/master
 checkpatch:
-	$Qeval COMMON_COMMIT=$$(git merge-base HEAD ${BASE_REF});	\
+	$QCOMMON_COMMIT=`git merge-base HEAD ${BASE_REF}`;		\
 	for commit in `git rev-list $$COMMON_COMMIT..HEAD`; do		\
 		echo "[*] Analyzing commit '$$commit'";			\
 		git format-patch --stdout "$$commit~..$$commit"		\
@@ -199,6 +201,11 @@ checkpatch:
 			| ${CHECKPATCH} - || true;			\
 	done
 
+# Target used to check for suspiciously missing changed files.
+
+checkdiff:
+	$Qcontrib/checkdiff.bash `git merge-base HEAD ${BASE_REF}`
+
 # This target is used during development in order to prevent adding new issues
 # to the source code. All warnings are treated as errors in order to block the
 # compilation and make the continous integration infrastructure return failure.