push: free_refs() the "local_refs" in set_refspecs()

Fix a memory leak that's been with us since this code was added in
ca02465b41 (push: use remote.$name.push as a refmap, 2013-12-03).

The "remote = remote_get(...)" added in the same commit would seem to
leak based only on the context here, but that function is a wrapper
for sticking the remotes we fetch into "the_repository->remote_state".

See fd3cb0501e (remote: move static variables into per-repository
struct, 2021-11-17) for the addition of code in repository.c that
free's the "remote" allocated here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5529-push-errors.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='detect some push errors early (before contacting remote)'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'setup commits' '