diffopts: notice a negated boolean flag in diffopts
This means that if you have git-diffs enabled by default (pretty
common) and you hit the rare (but real) case where a git-diff breaks
patch(1) or some other tool, you can easily disable it by just
specifying --no-git on the command line.
I feel a little bad about the isinstance() check, but some values in
diffopts are not booleans and so we need to preserve false iff the
flag is a boolean flag: failing to do this means we end up with empty
string defaults for flags clobbering meaningful values from the [diff]
section in hgrc.