rebase: hide --preserve-merges option

Since --preserve-merges has been deprecated in favour of
--rebase-merges, mark this option as hidden so it no longer shows up in
the usage and completions.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 21ac10f..0d63651 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1099,9 +1099,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			N_("let the user edit the list of commits to rebase"),
 			PARSE_OPT_NOARG | PARSE_OPT_NONEG,
 			parse_opt_interactive },
-		OPT_SET_INT('p', "preserve-merges", &options.type,
-			    N_("(DEPRECATED) try to recreate merges instead of "
-			       "ignoring them"), REBASE_PRESERVE_MERGES),
+		OPT_SET_INT_F('p', "preserve-merges", &options.type,
+			      N_("(DEPRECATED) try to recreate merges instead of "
+				 "ignoring them"),
+			      REBASE_PRESERVE_MERGES, PARSE_OPT_HIDDEN),
 		OPT_BOOL(0, "rerere-autoupdate",
 			 &options.allow_rerere_autoupdate,
 			 N_("allow rerere to update index with resolved "