revlog: add a _datareadfp context manager for data access needs
The helper handles:
1) is there a file handle already open that we shall just reuse,
2) is the revlog inlined or not.
Using a context manager for all read access will help setting up file pointer
caching 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)
}