Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

The rebase struct stores fields with information about the current
rebase process, which were not accessible via a public interface.

Accessors for getting the `orig_head` and `onto` branch
names and object ids have been added.

docs: clarify relation of safe and forced checkout strategy

remote: Rename git_remote_completion_type to _t

For consistency with other "type" enums, rename
git_remote_completion_type to git_remote_completion_t.

Custom backends can allocate memory when reading objects and providing
them to libgit2.  However, if an error occurs in the custom backend
after the memory has been allocated for the custom object but before
it's returned to libgit2, the custom backend has no way to free that
memory and it must be leaked.

Provide a free function that corresponds to the alloc function so that
custom backends have an opportunity to free memory before they return an
error.

The `git_odb_backend_malloc` name is a system function that is provided
for custom ODB backends and allows them to allocate memory for an ODB
object in the read callback.  This is important so that libgit2 can
later free the memory used by an ODB object that was read from the
custom backend.

However, the name _suggests_ that it actually allocates a
`git_odb_backend`.  It does not; rename it to make it clear that it
actually allocates backend _data_.

Safely deprecate `git_push_transfer_progress`, forwarding it to the new
`git_push_transfer_progress_cb` name.

The `git_push_transfer_progress` is a callback and as such should be
suffixed with `_cb` for consistency.  Rename
`git_push_transfer_progress` to `git_push_transfer_progress_cb`.