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>

t/t5510-fetch.sh

@@ -578,7 +578,7 @@ test_configured_prune () {
 			set_config_tristate fetch.prune $fetch_prune &&
 			set_config_tristate remote.origin.prune $remote_origin_prune &&
 
-			git fetch $cmdline &&
+			git fetch '"$cmdline"' &&
 			case "$expected_branch" in
 			pruned)
 				test_must_fail git rev-parse --verify refs/remotes/origin/newbranch