The signing callback should not be used; instead, callers should provide
a commit_create_cb, perform the signing and commit creation themselves.

Introduce a new mechanism for `git_rebase_commit` for callers to
customize the experience.  Instead of assuming that we produce the
commit for them, provide a commit creation callback that allows callers
to produce the commit themselves and return the resulting commit id.

Add sign capability to git_rebase_commit

Use ci_git_fail_with where appropriate.
Use correct initializer for callback.

In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

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.

In the case that we want to build merge + commit, cherrypick + commit, or even just build a commit with signing callback, `git_rebase_commit_signature_cb` particular callback should be made more generic. We also renamed `signature_cb` to `signing_cb` to improve clarity on the purpose of the callback (build a difference between a git_signature and the act of signing).

So we've ended up with `git_commit_signing_cb`.

Reduces the number of callbacks for signing a commit during a rebase operation to just one callback. That callback has 2 out git_buf parameters for signature and signature field. We use git_buf here, because we cannot make any assumptions about the heap allocator a user of the library might be using.