Incorrect tab-completion escaping with certain characters

[zx8]

$ fish --version
fish, version 3.0.2-948-g4ddfd730

Repro:

$ touch foo foo[bar]
$ ls foo[<TAB>
$ ls foo[bar\]

This should have completed to either foo[bar] or foo\[bar\], but not foo[bar\]:

[ridiculousfish]

I like the new approach better too! Merged as bc3dde9 . Thank you!!