closes#8362
Vim's code calls `call_shell` directly from `get_system_output_as_rettv`
whereas in Nvim this function has been rewritten to not call `call_shell` but to call
`os_system` via `do_os_system`, losing the support for profiling and verbose.
Changing the code to call `call_shell` from `get_system_output_as_rettv`
seems to be too complicated to be worth it on the current version of the
code. So this commit duplicates the relevant code.