reader_readline only ever reads from fish's STDIN

[faho]

Via gitter we got a question about a git commit-msg hook.

These run in an environment with a non-tty stdin, so if you want to query the user, you'd have to read </dev/tty explicitly. See e.g. https://stackoverflow.com/questions/3417896/how-do-i-prompt-the-user-from-within-a-commit-msg-hook.

Only that won't work in fish, because even tho read receives the redirected fd, it'll call reader_readline, and that has hard-coded 0 pretty extensively (down to input_event_queue_t::readb using FD_SET(0, &fdset)).

It would be great if this worked.

[mqudsi]

I've had to resort to using /dev/tty a lot in my fish scripts for a few other cases as well, but it's important to note that it's not just unfriendly but also unportable since I think it's a Linux-only thing.

[faho]

Sure, but:

1. This is about read reading interactively from anything, not just /dev/tty specifically
2. In this context you need to set the tty manually, so if you don't have /dev/tty you just need to pass it differently

I'm not asking for us to use this anywhere, I'm asking for us to make it possible.