filteredhash: split the computation of revision sets
The branch2's filteredhash combines the filtered revisions and the obsolete
ones, this will creates issue for implicit reference to heads we want to
introduce for the v3 of the branch cache format. So we isolate this logic for
alternative use.
{
// 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)
}