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

Fix a memory leak in code added in bf0231c661 (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>

submodule.c

@@ -2388,7 +2388,7 @@ int get_superproject_working_tree(struct strbuf *buf)
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct strbuf sb = STRBUF_INIT;
 	struct strbuf one_up = STRBUF_INIT;
-	const char *cwd = xgetcwd();
+	char *cwd = xgetcwd();
 	int ret = 0;
 	const char *subpath;
 	int code;
@@ -2451,6 +2451,7 @@ int get_superproject_working_tree(struct strbuf *buf)
 		ret = 1;
 		free(super_wt);
 	}
+	free(cwd);
 	strbuf_release(&sb);
 
 	code = finish_command(&cp);