Wrap function calls in the `checkout::crlf` tests with `cl_git_pass`,
`cl_assert`, etc. to ensure that they're successful.

After sandboxing the crlf directory, remove the working directory
contents.  This allows us to package data within the crlf directory
(for simplicity, this allows us to script the to-odb and to-workdir
crlf filter conversion data in a single location).

Move the crlf_data folders reponsible for holding the state of the
filters going into the working directory to "to_workdir" variations of
the folder name to accommodate future growth into the "to odb" filter
variation.  Update the script to create these new folders as appopriate.

When there's no matching index entry (for whatever reason), don't
try to dereference the null return value to get at the id.

Otherwise when we break something in the index API, the checkout
test crashes for confusing reasons and causes us to step through
it in a debugger thinking that we had broken much more than we
actually did.

All platforms do terrible, horrible, no good, very bad translation
when core.autocrlf=true.  It's not just Windows!

Given a variety of combinations of core.autocrlf settings and
attributes settings, test that we check out data into the working
directory the same as a known-good test resource created by git.git.

Include the UTF8 and UTF8 BOM tests in the master crlf test
branch for completeness.