submodule.c: free() memory from xgetcwd()

submodule.c: free() memory from xgetcwd()

Fix a memory leak in code added in bf0231c6614 (rev-parse: add
--show-superproject-working-tree, 2017-03-08), we should never have
made the result of xgetcwd() a "const char *", as we return a
strbuf_detach()'d value. Let's fix that and free() it when we're done
with it.

We can't mark any tests passing passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true" as a result of this change, but
e.g. "t/t1500-rev-parse.sh" now gets closer to passing.

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