rerere: Separate libgit and builtin functions

This patch moves rerere()-related functions into a newly created
rerere.c file.
The setup_rerere() function is needed by both rerere() and cmd_rerere(),
so this function is moved to rerere.c and declared non-static (and "extern")
in newly created rerere.h file.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.h

@@ -0,0 +1,9 @@
+#ifndef RERERE_H
+#define RERERE_H
+
+#include "path-list.h"
+
+extern int setup_rerere(struct path_list *);
+extern int rerere(void);
+
+#endif