fix(api): nvim_buf_set_text resets changelist to 0 #39965

Problem: changed_lines got a hardcoded 0, so the changelist entry
and '. mark always recorded column 0 instead of where the edit
actually happened.

Solution: pass start_col instead. changelist now tracks the real
column.

test/functional/api/buffer_spec.lua

@@ -975,6 +975,10 @@ describe('api/buf', function()
       eq('hello foo!', curbuf_depr('get_line', 0))
       -- cursor should be moved left by two columns (replacement is shorter by 2 chars)
       eq({ 1, 9 }, api.nvim_win_get_cursor(0))
+
+      -- changelist entry reflects the edit column #28618
+      local changes = fn.getchangelist()[1]
+      eq(6, changes[#changes].col)
     end)
 
     it('updates the cursor position in non-current window', function()