Problem: nvim_win_set_config may merge configs despite failing to configure a
split, and without applying necessary side-effects (like setting style=minimal
options). Plus, autocommands may apply a different config after the merge,
causing side-effects to apply for an outdated config.
Solution: merge configs last, only on success. Include fields only relevant to
splits. Properly set _cmdline_offset for splits.
Maybe better to disallow _cmdline_offset for splits instead, as the pum is
relative to cmdline_row anyway? (I didn't want to change behaviour too much)
Also use expect_unchanged in an unrelated test to quash a warning.
137d5ab01d