patch 8.2.2452: no completion for the 'filetype' option

Problem:    No completion for the 'filetype' option.
Solution:   Add filetype completion. (Martin Tournoij, closes #7747)
diff --git a/src/option.c b/src/option.c
index b4893a1..bba467e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2419,6 +2419,8 @@
 /*
  * Get a pointer to the flags used for the P_INSECURE flag of option
  * "opt_idx".  For some local options a local flags field is used.
+ * NOTE: Caller must make sure that "curwin" is set to the window from which
+ * the option is used.
  */
     static long_u *
 insecure_flag(int opt_idx, int opt_flags)
@@ -6209,6 +6211,10 @@
 	    else
 		xp->xp_backslash = XP_BS_ONE;
 	}
+	else if (p == (char_u *)&p_ft)
+	{
+	    xp->xp_context = EXPAND_FILETYPE;
+	}
 	else
 	{
 	    xp->xp_context = EXPAND_FILES;