upgrade: introduce a _copyrevlog method
This function copies a revlog from the old store to the new one, without
re-adding all deltas manually. This will eventually save a lot of time when some
revlog does not needs any conversions.
Code actually using this will be introduced in later changesets.
{
// 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)
}