lfs: allow a pointer to be extracted from a context that removes the file
This is needed to let 'set:lfs()' and '{lfs_files}' work normally on removed
files.
Yuya suggested returning a null pointer for removed files, instead of the
pointer from the parent. The first attempt at this was to return None for a non
LFS file, and a (pointer, ctx) tuple to hold the pointer and context (or parent
pointer and context for a removed file). But this complicated the callers, even
the ones that didn't care about removed files.
Instead, let's use {} to represent a removed pointer. This has the added
convenience of being a useful representation in the template language, and only
affects the callers that care about removed files (and only slightly). Since
pointers are explicitly serialized with a call to a member function, there is no
danger of writing these to disk.
{
// 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)
}