fish vi-mode: Fix initial undo failure
fish vi-mode: Fix initial undo failure

Since we go straight into insert mode when starting vi-mode, we haven't
called __vi_mode_save_cmdline yet and trying to do an undo after leaving
insert mode will fail.

Initialise the undo variables when loading vi-mode.

Also, add the __vi_mode_ prefix to these variables, which I missed
before.

Reported-by: Maxim Gonchar <da68200212665202b77b80ac3713a421ca9a0788@gmail.com>
Signed-off-by: Ian Munsie <ce389fd2fa887d610219cb85dea6dc9451973e74@gmail.com>

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