Always reference master branch for incremental subjects

https://github.com/mbj/mutant/blob/main/docs/incremental.md#recommended-use

HEAD~1 seems to be too narrow selector, referencing previous commit.
That's effectively no mutation when you open a new PR.

Author: mostlyobvious

support/make/mutant.mk

@@ -8,4 +8,4 @@ mutate-fast: ## Run mutation tests with --fail-fast
 
 mutate-changes: ## Run incremental mutation tests
 	@echo "Running mutation tests"
-	@bundle exec mutant run --since HEAD~1 $(SUBJECT)
+	@bundle exec mutant run --since master $(SUBJECT)