patch 9.1.0132: "C" doesn't include composing chars with 'virtualedit'

Problem:  using "C" and 've=all' set, doesn't include composing chars
          when changing a line, keeps the composing chars for whatever
          is typed afterwards.
Solution: Use mb_head_off() and mb_ptr2len() instead of mb_tail_off().
          (zeertzjq)

closes: #14083

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_virtualedit.vim

@@ -77,13 +77,30 @@ endfunc
 func Test_edit_change()
   new
   set virtualedit=all
+
   call setline(1, "\t⒌")
   normal Cx
   call assert_equal('x', getline(1))
+
+  call setline(1, "\ta̳")
+  normal Cx
+  call assert_equal('x', getline(1))
+
+  call setline(1, "\tβ̳")
+  normal Cx
+  call assert_equal('x', getline(1))
+
+  if has('arabic')
+    call setline(1, "\tلا")
+    normal Cx
+    call assert_equal('x', getline(1))
+  endif
+
   " Do a visual block change
   call setline(1, ['a', 'b', 'c'])
   exe "normal gg3l\<C-V>2jcx"
   call assert_equal(['a  x', 'b  x', 'c  x'], getline(1, '$'))
+
   bwipe!
   set virtualedit=
 endfunc