rerere forget path: forget recorded resolution

After you find out an earlier resolution you told rerere to use was a
mismerge, there is no easy way to clear it.  A new subcommand "forget" can
be used to tell git to forget a recorded resolution, so that you can redo
the merge from scratch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.h

@@ -7,5 +7,6 @@ extern int setup_rerere(struct string_list *);
 extern int rerere(void);
 extern const char *rerere_path(const char *hex, const char *file);
 extern int has_rerere_resolution(const char *hex);
+extern int rerere_forget(const char **);
 
 #endif