remove obsolete index-reuc-entry type
remove git_diff_blobs binding (for now)

Its specification changed in libgit 0.19.0.
remove reference-type arguments

They were removed in libgit 0.19.0.
use git_reference_foreach_name in references-fold

`git_reference_foreach` (used by the Scheme API in `references-fold`)
used to yield reference names, but now (as of libgit 0.19.0) yields
references, so this changes `references-fold` to use the new
`git_reference_foreach_name` (until I have a chance to verify the memory
management semantics of the new functions).
remove remote push/fetchspec bindings

The API was redesigned in libgit2 0.19.0 and the current bindings no
longer work correctly.
config-open no longer errors on nonexistent files, fix test