patch 9.0.0414: matchstr() still does not match column offset

Problem:    matchstr() still does not match column offset when done after a
            text search.
Solution:   Only use the line number for a multi-line search.  Fix the test.
            (closes #10938)

src/testdir/test_regexp_latin.vim

@@ -1145,7 +1145,13 @@ def Test_compare_columns()
 enddef
 
 def Test_compare_column_matchstr()
+  # do some search in text to set the line number, it should be ignored in
+  # matchstr().
   enew
+  setline(1, ['one', 'two', 'three'])
+  :3 
+  :/ee
+  bwipe!
   set re=1
   call assert_equal('aaa', matchstr('aaaaaaaaaaaaaaaaaaaa', '.*\%<5v'))
   set re=2