Make git-mv a builtin

This also moves add_file_to_index() to read-cache.c. Oh, and while
touching builtin-add.c, it also removes a duplicate git_config() call.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/cache.h b/cache.h
index d0b1f27..c575b8a 100644
--- a/cache.h
+++ b/cache.h
@@ -154,6 +154,7 @@
 extern struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int really);
 extern int remove_cache_entry_at(int pos);
 extern int remove_file_from_cache(const char *path);
+extern int add_file_to_index(const char *path, int verbose);
 extern int ce_same_name(struct cache_entry *a, struct cache_entry *b);
 extern int ce_match_stat(struct cache_entry *ce, struct stat *st, int);
 extern int ce_modified(struct cache_entry *ce, struct stat *st, int);