Very many callbacks in libgit2 have some sort of typedef to normalize the name at git_<name_of_operation>_cb. Add a typedef for push_update_references in the remote so the name follows the same conventions.

patch_generate: represent buffers as void pointers

Remove unused 'sys/remote.h' header

Patch ID calculation

Pointers to general data should usually be used as a void pointer such
that it is possible to hand in variables of a different pointer type
without the need to cast. This is the same when creating patches from
buffers, where the buffers may contain arbitrary data. Instead of
requiring the caller to care whether his buffer is e.g. `char *` or
`unsigned char *`, we should instead just accept a `void *`. This is
also consistent in how we tread other types like for example `git_blob`,
which also just has a void pointer as its raw contents.