Long story short: the SSH agent protocol doesn't support telling from
which tty the request is coming from, so the the pinentry curses prompt
appears on the login tty and messes up the output and may hang.
The current trick to workaround this is informing the gnupg agent every
time you start a shell: this assumes you will run `ssh` in the latest
tty, if you don't the latest tty will be messed up this time.
The ideal solution would be updating the tty exactly when (and where)
you run `ssh`. This is actually possible using a catch-all Match block
in ssh_config and using the `exec` feature that hooks a command to the
current shell.
Source for the new trick: https://unix.stackexchange.com/a/499133/110465
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