t2004: demonstrate broken relative path printing

t2004: demonstrate broken relative path printing

checkout-index --temp only properly prints relative paths which are
descendants of the current directory. Paths in ancestor or sibling
directories (or their children) are often printed in mangled form. For
example:

    mkdir a bbb &&
    >file &&
    >bbb/file &&
    git update-index --add file bbb/file &&
    cd a &&
    git checkout-index --temp ../file ../bbb/file

prints:

    .merge_file_ooblek  le
    .merge_file_igloo0  b/file

rather than the correct:

    .merge_file_ooblek  ../file
    .merge_file_igloo0  ../bbb/file

Unfortunately, testing is complicated slightly by relative paths
sometimes _appearing_ to be printed correctly, but this is an accident
of implementation in which a "correct" copy of the string exists in
memory beyond the end of the real string, and that "correct" copy gets
printed. This test takes care to avoid the accidentally "correct"
behavior by testing with a filename longer than the directory name in
which checkout-index is invoked.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>