sparse-index: recompute cache-tree

When some commands run with command_requires_full_index=1, then the
index can get in a state where the in-memory cache tree is actually
equal to the sparse index's cache tree instead of the full one.

This results in incorrect entry_count values. By clearing the cache
tree before converting to sparse, we avoid this issue.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

sparse-index.c

@@ -170,6 +170,8 @@ int convert_to_sparse(struct index_state *istate)
 	if (index_has_unmerged_entries(istate))
 		return 0;
 
+	/* Clear and recompute the cache-tree */
+	cache_tree_free(&istate->cache_tree);
 	if (cache_tree_update(istate, 0)) {
 		warning(_("unable to update cache-tree, staying full"));
 		return -1;