Problem:
After expanding tabs with the `expandtab` option, if the old indent
matches the requested one, `vim.text.indent()` returns the input
unchanged.
The optimization path assumes that if old_indent == size, the input
wouldn't be changed, which is not correct when old_indent is formed by
expanded tabs.
Solution:
Apply the optimization only when `expandtab` is not set.
dec3d8215d
