ident.h: move ident-related declarations out of cache.h

These functions were all defined in a separate ident.c already, so
create ident.h and move the declarations into that file.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -10,6 +10,7 @@
 #include "builtin.h"
 #include "diff.h"
 #include "hex.h"
+#include "ident.h"
 #include "parse-options.h"
 #include "userdiff.h"
 #include "streaming.h"