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

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>