Call fish_should_add_to_history to see if a command should be saved
If it returns 0, it will be saved, if it returns anything else, it
will be ephemeral.
It gets the right-trimmed text as the argument.
If it doesn't exist, we do the historical behavior of checking for a
leading space.
That means you can now turn that off by defining a
`fish_should_add_to_history` that just doesn't check it.
documentation based on #9298
f7cc1743c6