Fixes an error raised by cvs test with Postgresql (revision is varchar).
scm: cvs: add invalid revision annotate test in unit model test.
scm: cvs: add test annotate with revision in unit model test.
scm: cvs: use scm_cmd() in annotate of adapter.
scm: cvs: add annotate test of HEAD revision in unit model test.
scm: cvs: override annotate() in model.
scm: cvs: add test that cat returns nil if revision does not exist at model.
scm: cvs: cat returns nil if revision does not exist at model.
scm: cvs: add cat test of removed file in unit model test.
scm: cvs: code clean up model.
