the resulting commit id
the repository to create the commit in.
the content of the unsigned commit object
the signature to add to the commit. Leave null to create a commit without adding a signature field.
which header field should contain this signature. Leave null for the default of "gpgsig"
0 or an error code
Create a commit object from the given buffer and signature
Given the unsigned commit object's contents, its signature and the header field in which to store the signature, attach the signature to the commit and write it into the given repository.