fish -c "complete -C" messes with terminal modes

[krobelus]

Run

fish -c 'complete -C ": "' | sed 1q | vim -

sometimes vim ends up in a broken state: characters are echoed as I type them.

It does not happen if stdin is not a terminal:

echo | fish -c 'complete -C ": "' | sed 1q | vim -u NONE -

Looks like fish restores terminal modes, which overwrites any modes set by vim.

write(1, "\n", 1)                       = -1 EPIPE (Broken pipe)
--- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=1027314, si_uid=1000} ---
getpid()                                = 1027314
getpgrp()                               = 1027309
ioctl(0, TIOCGPGRP, [1027309])          = 0
ioctl(0, SNDCTL_TMR_START or TCSETS, {B38400 opost isig icanon echo ...}) = 0

Sort of related to #8699 - but the fix is probably different.
We can just not set any terminal modes in this scenario.