Update Makefile

Author: staabm

Makefile

@@ -178,7 +178,7 @@ infection:
 		wget https://github.com/infection/infection/releases/download/$$version/infection.phar --timestamping;
 	php infection.phar \
 		--configuration=infection.json5 \
-		--git-diff-base=origin/2.1.x \
+		--git-diff-base=origin/HEAD \
 		--git-diff-lines \
 		--coverage=tmp/coverage \
 		--skip-initial-tests \