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/var.c

@@ -5,6 +5,7 @@
  */
 #include "builtin.h"
 #include "config.h"
+#include "ident.h"
 #include "refs.h"
 
 static const char var_usage[] = "git var (-l | <variable>)";