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.

If provided with a null signature, skip adding the signature header and create the commit anyway.

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`.

Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.

The functions `git_commit_header_field` and
`git_commit_extract_signature` both receive buffers used to hand back
the results to the user. While these functions called `git_buf_sanitize`
on these buffers, this is not the right thing to do, as it will simply
initialize or zero-terminate passed buffers. As we want to overwrite
contents, we instead have to call `git_buf_clear` to completely reset
them.

In combination with the function which creates a commit into a buffer,
this allows us to more easily create signed commits.