dirstate: do not write an empty dirstate just for backup
This will get in the way when we get more strict about holding the lock when
writing the dirstate.
Instead, we simply don't copy dirstate files around if there are None at backup
time.
A couple of tests are impacted they no longer need to backup such "empty"
dirstate.
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}