[sledge] Clear terminal between builds in watch mode

Summary:
Otherwise it is difficult to tell the difference between compilation
errors from previous versus current builds.

Reviewed By: ngorogiannis

Differential Revision: D18736376

fbshipit-source-id: 2e583f4ba

sledge/Makefile

@@ -63,7 +63,7 @@ opt: setup
 
 .PHONY: watch
 watch: setup
-	dune build --watch $(TARGETS)
+	dune build --watch --terminal-persistence=clear-on-rebuild $(TARGETS)
 
 .PHONY: test
 test: setup