[PATCH] Make tests more portable

I made some minor changes to the test suite to make the tests more portable.  The sed on
Darwin doesn't understand extended regex, cmp won't read from '-', and xargs doesn't have
an '-r' command line flag.

The t3000 test was broken because it wasn't updated when Linus merged Junio's patch to
make git-ls-files show filenames with leading dots.  I fixed that with a trivial
addition. 

Cheers,

--Mark