t2*checkout* official tests

[sirnewton01]

If the majority of these official tests pass then we can be more confident that checkout and branching is working properly.

[driusan]

The current status of this is:

[x] t2000-checkout-cache-clash.sh
[x] t2001-checkout-cache-clash.sh
[x] t2002-checkout-cache-u.sh
[ ] t2003-checkout-cache-mkdir.sh
[ ] t2004-checkout-cache-temp.sh
[ ] t2005-checkout-index-symlinks.sh
[x] t2006-checkout-index-basic.sh
[ ] t2007-checkout-symlink.sh
[ ] t2008-checkout-subdir.sh
[x] t2009-checkout-statinfo.sh
[ ] t2010-checkout-ambiguous.sh
[ ] t2011-checkout-invalid-head.sh
[ ] t2012-checkout-last.sh
[ ] t2013-checkout-submodule.sh
[x] t2014-switch.sh
[ ] t2015-checkout-unborn.sh
[ ] t2016-checkout-patch.sh
[ ] t2017-checkout-orphan.sh
[x] t2018-checkout-branch.sh
[ ] t2019-checkout-ambiguous-ref.sh
[ ] t2020-checkout-detach.sh
[ ] t2021-checkout-overwrite.sh
[ ] t2022-checkout-paths.sh
[ ] t2023-checkout-m.sh
[ ] t2024-checkout-dwim.sh