remove --boundary from %(cmdlineargs)

If you ask tig to show boundary commits, like:

    tig --boundary origin...master

we end up passing `--boundary` along to `git show` when we open the diff view for the commit. As a result, each diff contains not only the commit we cared about, but also the commit message and diff for its parent.

We can fix this by adding `--boundary` to the list of revision args to omit in `filter_options`.