patch 9.1.0843: too many strlen() calls in undo.c

patch 9.1.0843: too many strlen() calls in undo.c

Problem:  too many strlen() calls in undo.c
Solution: refactor code and remove strlen() calls, update test_undo.vim
          and close remaining open swap files (John Marriott)

closes: #15995

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>