evolve: protect call to rebase within a wlock (#42, #35, #16) Without a wlock, repo.commit would blow away the dirstate's parents on OSes that have no 'os.symlink' support in python, leading evolve to produce a merge instead of a rebase. If a user ran the rebase command instead of evolve, then things would work because rebase is wrapped in a giant wlock. Unfortunately, we can't use the same idea of wrapping the evolve command in one giant wlock because that's too early in the process. If the lock did wrap the entire evolve command, then the working directory would save its current parents which, since rebase hasn't been called yet, would be just p1. Therefore, we need to obtain the lock *after* the dirstate's parents are changed but *before* the call to rebase. This way ensures that when a conflict happens the working directory correctly shows both parent changeset.

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