checkout test: enable test with complex relative path

checkout test: enable test with complex relative path

This test was added, commented out, in fed1b5ca (git-checkout: Test
for relative path use, 2007-11-09).  Later git's path handling was
improved (d089ebaa, setup: sanitize absolute and funny paths in
get_pathspec(), 2008-01-28) but we forgot to enable the now-working
test.

This test expects to run from a subdirectory, so add a 'cd'.  While
we're here, examine the content of the checked-out file instead of
just checking that it exists.  The other checkout tests already do the
same.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>