parallel-checkout: avoid dash local bug in tests

Dash bug https://bugs.launchpad.net/ubuntu/+source/dash/+bug/139097
lets the shell erroneously perform field splitting on the expansion of a
command substitution during declaration of a local variable.  It causes
the parallel-checkout tests to fail e.g. when running them with
/bin/dash on MacOS 11.4, where they error out like this:

   ./t2080-parallel-checkout-basics.sh: 33: local: 0: bad variable name

That's because the output of wc -l contains leading spaces and the
returned number of lines is treated as another variable to declare, i.e.
as in "local workers= 0".

Work around it by enclosing the command substitution in quotes.

Helped-by: Matheus Tavares Bernardino <matheus.bernardino@usp.br>
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-parallel-checkout.sh

@@ -27,7 +27,7 @@ test_checkout_workers () {
 	rm -f "$trace_file" &&
 	GIT_TRACE2="$(pwd)/$trace_file" "$@" 2>&8 &&
 
-	local workers=$(grep "child_start\[..*\] git checkout--worker" "$trace_file" | wc -l) &&
+	local workers="$(grep "child_start\[..*\] git checkout--worker" "$trace_file" | wc -l)" &&
 	test $workers -eq $expected_workers &&
 	rm "$trace_file"
 } 8>&2 2>&4