Commit signing callback.

The callback will be called with the commit content, giving a user an opportunity to sign the commit content. The signature_field buf may be left empty to specify the default field "gpgsig".

Signatures can take the form of any string, and can be created on an arbitrary header field. Signatures are most commonly used for verifying authorship of a commit using GPG or a similar cryptographically secure signing algorithm. See https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work for more details.

When the callback:

• returns GIT_PASSTHROUGH, no signature will be added to the commit.
• returns < 0, commit creation will be aborted.
• returns GIT_OK, the signature parameter is expected to be filled.