index-pack: retain child bases in delta cache

When resolving a delta whose result has children of its own,
index-pack adds the result to work_head, accounts its data in
base_cache_used, and calls prune_base_data(). It then immediately
frees that same data.

This bypasses the existing delta base cache policy and can force later
descendants to reconstruct the queued base again. Let the existing
delta_base_cache_limit pruning policy decide whether to keep or evict
the data instead.

Signed-off-by: Arijit Banerjee <arijit@effectiveailabs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Arijit Banerjee

builtin/index-pack.c

@@ -1212,7 +1212,6 @@ static void *threaded_second_pass(void *data)
 			list_add(&child->list, &work_head);
 			base_cache_used += child->size;
 			prune_base_data(NULL);
-			free_base_data(child);
 		} else if (child) {
 			/*
 			 * This child does not have its own children. It may be