[sledge] Only fmt one (dbg) context, and suppress diff output

Summary:
`dune build fmt` applies to all contexts, but we only need one, so
use `dune build @_build/dbg/fmt` instead. Also swallow stderr as it's
useless with `--auto-promote`.

Reviewed By: ngorogiannis

Differential Revision: D20482767

fbshipit-source-id: a7642f65d

sledge/Makefile

@@ -55,7 +55,7 @@ watch:
 .PHONY: test
 test:
 	-dune build @_build/dbg/runtest --auto-promote
-	dune build @fmt --auto-promote
+	dune build @_build/dbg/fmt --auto-promote 2>/dev/null
 
 .PHONY: ci-test
 ci-test:
@@ -67,7 +67,7 @@ clean:
 
 .PHONY: fmt
 fmt:
-	dune build @fmt --auto-promote
+	dune build @_build/dbg/fmt --auto-promote 2>/dev/null
 	clang-format -i model/llair_intrinsics.h model/cxxabi.cpp
 	${MAKE} -C test fmt