build: add working lint-ci make target

PR-URL: #9151
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>

Makefile

@@ -585,6 +585,17 @@ cpplint:
 
 lint: jslint cpplint
 
+CONFLICT_RE=^>>>>>>> [0-9A-Fa-f]+|^<<<<<<< [A-Za-z]+
+lint-ci: jslint cpplint
+	@if ! ( grep -IEqrs "$(CONFLICT_RE)" benchmark deps doc lib src test tools ) \
+		&& ! ( find . -maxdepth 1 -type f | xargs grep -IEqs "$(CONFLICT_RE)" ); then \
+		exit 0 ; \
+	else \
+		echo "" >&2 ; \
+		echo "Conflict marker detected in one or more files. Please fix them first." >&2 ; \
+		exit 1 ; \
+	fi
+
 .PHONY: lint cpplint jslint bench clean docopen docclean doc dist distclean \
 	check uninstall install install-includes install-bin all staticlib \
 	dynamiclib test test-all test-addons build-addons website-upload pkg \