dirstate-v2: add devel config option to control write behavior This will help us to write predictable tests checking behavior in each case.

dirstate: use more than a bool to control append behavior When writing dirstate-v2, we might either append to the existing file, or create a new file. We are about to introduce some configuration to control this behavior. As a prelude, we change the current way the behavior was automatically controlled to make the change smaller/clearer.

dirstate: cover each dirstate version when testing for status race Previously we were only testing it with the default (dirstate-v1 currently). Now we explicitly test each variant.

dirstate: test a `hg status` raced by another `hg status` This shows that `rhg` is misbehaving here.

dirstate: test a `hg status` raced by a `hg update` This shows that `rhg` is misbehaving here.

dirstate: test a `hg status` raced by a `hg commit` This shows that `rhg` is misbehaving here.