unpack-trees: free cache_entry array members for merges

unpack-trees: free cache_entry array members for merges

The merge functions duplicate entries as needed and they don't free
them.  Release them in unpack_nondirectories, the same function
where they were allocated, after we're done.

As suggested by Felipe, use the same loop style (zero-based for loop)
for freeing as for allocating.

Improved-by: Felipe Contreras <redacted>
Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>