patch 9.1.1145: multi-line completion has wrong indentation for last line

Problem:  When expanding omni completion items with newlines (e.g.
          `then\n\t\nend`), the end statement gets wrong indentation.
Solution: Add OPENLINE_FORCE_INDENT flag to make open_line() use
          second_line_indent directly (glepnir)

closes: #16614

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Justin M. Keyes <justinkz@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/change.c b/src/change.c
index 294adf7..a3329bd 100644
--- a/src/change.c
+++ b/src/change.c
@@ -1385,6 +1385,8 @@
  *	    OPENLINE_KEEPTRAIL	keep trailing spaces
  *	    OPENLINE_MARKFIX	adjust mark positions after the line break
  *	    OPENLINE_COM_LIST	format comments with list or 2nd line indent
+ *	    OPENLINE_FORCE_INDENT  set indent from second_line_indent, ignore
+ *				   'autoindent'
  *
  * "second_line_indent": indent for after ^^D in Insert mode or if flag
  *			  OPENLINE_COM_LIST
@@ -1498,9 +1500,11 @@
     if (dir == FORWARD && did_ai)
 	trunc_line = TRUE;
 
+    if ((flags & OPENLINE_FORCE_INDENT) && second_line_indent >= 0)
+	newindent = second_line_indent;
     // If 'autoindent' and/or 'smartindent' is set, try to figure out what
     // indent to use for the new line.
-    if (curbuf->b_p_ai || do_si)
+    else if (curbuf->b_p_ai || do_si)
     {
 	// count white space on current line
 #ifdef FEAT_VARTABS