fetch tests: double quote a variable for interpolation

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>