reset: free allocated tree buffers

reset: free allocated tree buffers

We read the tree objects with fill_tree_descriptor(), but
never actually free the resulting buffers, causing a memory
leak. This isn't a huge deal because we call this code at
most twice per program invocation. But it does potentially
double our heap usage if you have large root trees. Let's
free the trees before returning.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>