fetch tests: double quote a variable for interpolation

If the $cmdline variable contains arguments with spaces they won't be interpolated correctly, since the body of the test is single quoted, and because test-lib.sh does its own eval(). This will be used in a subsequent commit to pass arguments that need to be quoted to git-fetch, i.e. a file:// path to fetch, which will have a space in it. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>