Add some tests of git-cvsimport's handling of vendor branches

Add some tests of git-cvsimport's handling of vendor branches

CVS's handling of vendor branches is tricky; add some tests to check
whether revisions added via "cvs imports" then imported to git via
"git cvsimport" are reflected correctly on master.

One of these tests fail and is therefore marked "test_expect_failure".
Cvsimport doesn't realize that subsequent changes on a vendor branch
affect master as long as the vendor branch is the default branch.

The test CVS repository used for these tests is derived from cvs2svn's
test suite.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>