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.
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