dirstate: avoid leaking disk space in `hg debugrebuilddirstate`
Before this MR running `hg debugrebuilddirstate` simply grows the dirstate
without bound, never shrinking it, because the unused bytes counter stays low,
even though the entirety of the file becomes unused.
{
// 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)
}