Bug#713922: sh.vim is confused by "$(cmd --)"

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Package: vim-runtime
Version: 2:7.3.923-2

I ran into this on line 289 of /var/lib/dpkg/info/libc6:i386.preinst:
    if [ "${DPKG_MAINTSCRIPT_ARCH:-$(dpkg --print-architecture)}" = "armhf" ]; then

All lines after this have the syntax messed up, apparently due to losing
track of the double-quote state.

Although $() is involved, this is not Bug#712657, because I have b:is_bash
set, and $() is recognized in other contexts.

The key thing appears to be the "--" in the subcommand.
I can reduce it to:

    #!/bin/bash
    echo "$(foo --) this should be purple"
    : and this line should not

But the quotes, the $(), and the -- all appear to be necessary.