t0001: handle `diff --no-index` gracefully

t0001: handle `diff --no-index` gracefully

The test case 're-init to move gitdir symlink' wants to compare the
contents of `newdir/.git`, which is a symbolic link pointing to a file.
However, `git diff --no-index`, which is used by `test_cmp` on Windows,
does not resolve symlinks; It shows the symlink _target_ instead (with a
file mode of 120000). That is totally unexpected by the test case, which
as a consequence fails, meaning that it's a bug in the test case itself.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>