Add missing options to b:undo_ftplugin

[dkearns]

The bzl ftplugin inherits this variable from the sourced python ftplugin but doesn't restore the options it exclusively sets.

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[dkearns]

@googlebot I signed it!

[dbarnett]

Isn't b:undo_ftplugin not necessarily defined yet, in which case this would fail?

[dkearns]

It's defined in the sourced Python ftplugin sourced at line 43.

[dbarnett]

Oh, right! Could you put a comment above this line to note it's initially defined in ftplugin/python.vim?

[dkearns]

Done

[dbarnett]

Looks good, thanks! Merging.