Commit signing callback: used when a function is going to write
a commit (for example, in git_commit_create_ext) to allow callers
to sign the commit.
the raw contents of the commit (to be signed)
the caller-specified callback payload
0 if this callback has created a signature and populated the field and signature buffers, GIT_PASSTHROUGH if the callback does not want to sign the commit, any other value to stop and return a failure