build: add conflict marker check during CI lint

PR-URL: #7625
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Makefile

@@ -706,7 +706,16 @@ cpplint:
 
 ifneq ("","$(wildcard tools/eslint/bin/eslint.js)")
 lint: jslint cpplint
+CONFLICT_RE=^>>>>>>> [0-9A-Fa-f]+|^<<<<<<< [A-Za-z]+
 lint-ci: jslint-ci cpplint
+	@if ! ( grep -IEqrs "$(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
 else
 lint:
 	@echo "Linting is not available through the source tarball."