dir.c: free removed sparse-pattern hashmap entries

In add_pattern_to_hashsets(), we remove entries from the
recursive_hashmap when adding similar ones to the parent_hashmap. I
won't pretend to understand all of what's going on here, but there's an
obvious leak: whatever we removed from recursive_hashmap is not
referenced anywhere else, and is never free()d.

We can easily fix this by asking the hashmap to return a pointer to the
old entry. This makes t7002 now completely leak-free.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7002-mv-sparse-checkout.sh

@@ -2,6 +2,7 @@
 
 test_description='git mv in sparse working trees'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 setup_sparse_checkout () {