Commit to buff instead of object database (ODB) before signing

[gchronis]

Currently, in order to create signed commit, kevlar-laces creates an unsigned commit and stores it in the ODB, then amends it and adds a signature to the header, returning the second commit. This leaves the repo littered with unused objects. To fix this, we want to use the libgit2 function commit_to_buf, which creates the commit without saving it to the object database first, and then use the commit_signed gitrs method to add the signature and write the commit to the ODB.

The bindings for this particular function have not been added to git2rs.