Add support for marking files no-reformat.

mk/pp.mk

@@ -4,9 +4,14 @@ PP_INPUTS := $(wildcard $(addprefix $(S)src/lib/,*.rs */*.rs)) \
              $(wildcard $(S)src/test/*/*.rs   \
                         $(S)src/test/*/*/*.rs)
 
+PP_INPUTS_FILTERED = $(shell echo $(PP_INPUTS) | xargs grep -L no-reformat)
+
 reformat: $(SREQ1)
 	@$(call E, reformat [stage1]: $@)
-	for i in $(PP_INPUTS);  \
+	for i in $(PP_INPUTS_FILTERED);  \
     do $(call CFG_RUN_TARG,stage1, stage1/rustc$(X)) \
-       --pretty normal $$i >$$i.tmp && mv $$i.tmp $$i; \
+       --pretty normal $$i >$$i.tmp; \
+    if cmp --silent $$i.tmp $$i; \
+        then echo no changes to $$i; rm $$i.tmp; \
+        else mv $$i.tmp $$i; fi \
     done

src/test/run-pass/block-expr-precedence.rs

@@ -1,3 +1,5 @@
+// no-reformat
+
 /*
  *
  *  When you write a block-expression thing followed by