testing/guile-git: fix further incompatibilities with libgit2 1.7.0
Also add a custom Scheme script which aims to detect incompatibles with future libgit2 versions by perform sanitychecks on the bytestructure descriptors.
Also add a custom Scheme script which aims to detect incompatibles with future libgit2 versions by perform sanitychecks on the bytestructure descriptors.