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>

builtin-commit.c

@@ -22,6 +22,7 @@
 #include "utf8.h"
 #include "parse-options.h"
 #include "path-list.h"
+#include "rerere.h"
 #include "unpack-trees.h"
 
 static const char * const builtin_commit_usage[] = {