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>
diff --git a/ident.c b/ident.c
index 6de76f9..8fad92d 100644
--- a/ident.c
+++ b/ident.c
@@ -5,10 +5,13 @@
  *
  * Copyright (C) 2005 Linus Torvalds
  */
-#include "cache.h"
+#include "git-compat-util.h"
+#include "ident.h"
 #include "config.h"
 #include "date.h"
+#include "gettext.h"
 #include "mailmap.h"
+#include "strbuf.h"
 
 static struct strbuf git_default_name = STRBUF_INIT;
 static struct strbuf git_default_email = STRBUF_INIT;