checkout: add tests for -b and --track

checkout: add tests for -b and --track

Test git checkout -b with and without --track and demonstrate unexpected
error messages when it's given an extra (i.e. unsupported) path
argument.  In both cases it reports:

   $ git checkout -b foo origin/master bar
   fatal: 'bar' is not a commit and a branch 'foo' cannot be created from it

The problem is that the start point we gave for the new branch is
"origin/master" and "bar" is just some extra argument -- it could even
be a valid commit, which would make the message even more confusing.  We
have more fitting error messages in git commit, but get confused; use
the text of the rights ones in the tests.

Reported-by: Dana Dahlstrom <dahlstrom@google.com>
Original-test-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>