patch 9.1.1389: completion: still some issue when 'isexpand' contains a space

Problem:  Cannot get completion startcol when space is not the first
          trigger character (after v9.1.1383)
Solution: Detect the next comma followed by a space in the option string
          and use in next compare loop (glepnir)

closes: #17311

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index 741cc9d..8bce0e4 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -4507,6 +4507,13 @@
   set ise=\ ,=
   call feedkeys("Sif true  \<ESC>:let g:result=complete_match()\<CR>", 'tx')
   call assert_equal([[8, ' ']], g:result)
+  call feedkeys("Slet a = \<ESC>:let g:result=complete_match()\<CR>", 'tx')
+  call assert_equal([[7, '=']], g:result)
+  set ise={,\ ,=
+  call feedkeys("Sif true  \<ESC>:let g:result=complete_match()\<CR>", 'tx')
+  call assert_equal([[8, ' ']], g:result)
+  call feedkeys("S{ \<ESC>:let g:result=complete_match()\<CR>", 'tx')
+  call assert_equal([[1, '{']], g:result)
 
   bw!
   unlet g:result