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/insexpand.c b/src/insexpand.c
index deae1ce..edc4265 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -4473,6 +4473,7 @@
 {
     char_u	*start = str;
     char_u	*curr = str;
+    int		base_indent = get_indent();
 
     while (*curr != NUL)
     {
@@ -4483,7 +4484,7 @@
 		ins_char_bytes(start, (int)(curr - start));
 
 	    // Handle newline
-	    open_line(FORWARD, OPENLINE_KEEPTRAIL, FALSE, NULL);
+	    open_line(FORWARD, OPENLINE_KEEPTRAIL | OPENLINE_FORCE_INDENT, base_indent, NULL);
 	    start = curr + 1;
 	}
 	curr++;