rebase: stop mentioning the -p option in comments

We no longer support `--preserve-merges`, therefore it does not make
sense to keep mentioning that option, even in code comments.

Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -909,7 +909,7 @@ static int parse_opt_merge(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
-/* -i followed by -p is still explicitly interactive, but -p alone is not */
+/* -i followed by -r is still explicitly interactive, but -r alone is not */
 static int parse_opt_interactive(const struct option *opt, const char *arg,
 				 int unset)
 {