statinfo: move stat_{data,validity} functions from cache/read-cache

These functions do not depend upon struct cache_entry or struct
index_state in any way, and it seems more logical to break them out into
this file, especially since statinfo.h already has the struct stat_data
declaration.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index b09c816..339c298 100644
--- a/Makefile
+++ b/Makefile
@@ -1142,6 +1142,7 @@
 LIB_OBJS += sparse-index.o
 LIB_OBJS += split-index.o
 LIB_OBJS += stable-qsort.o
+LIB_OBJS += statinfo.o
 LIB_OBJS += strbuf.o
 LIB_OBJS += streaming.o
 LIB_OBJS += string-list.o