patch 9.0.2025: no cmdline completion for ++opt args

Problem:  no cmdline completion for ++opt args
Solution: Add cmdline completion for :e ++opt=arg and :terminal
          [++options]

closes: #13319

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>
diff --git a/src/optionstr.c b/src/optionstr.c
index 202e93e..8458f2a 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -2105,6 +2105,19 @@
 }
 
 /*
+ * Function given to ExpandGeneric() to obtain the possible arguments of the
+ * fileformat options.
+ */
+    char_u *
+get_fileformat_name(expand_T *xp UNUSED, int idx)
+{
+    if (idx >= (int)ARRAY_LENGTH(p_ff_values))
+	return NULL;
+
+    return (char_u*)p_ff_values[idx];
+}
+
+/*
  * The 'fileformats' option is changed.
  */
     char *