* guix/git.scm (commit-closure): Add 'visited' optional parameter. (commit-difference): Add 'excluded' optional parameter; pass second argument to 'commit-closure'. * tests/git.scm ("commit-difference, excluded commits"): New test.
* guix/git.scm (commit-closure, commit-difference): New procedures. * guix/tests/git.scm, tests/git.scm: New files. * Makefile.am (dist_noinst_DATA): Add guix/tests/git.scm. (SCM_TESTS): Add tests/git.scm.