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.
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1