The delta-base-cache mechanism has been a key to the performance in
a repository with a tightly packed packfile, but it did not scale
well even with a larger value of core.deltaBaseCacheLimit.
* jk/delta-base-cache:
t/perf: add basic perf tests for delta base cache
delta_base_cache: use hashmap.h
delta_base_cache: drop special treatment of blobs
delta_base_cache: use list.h for LRU
release_delta_base_cache: reuse existing detach function
clear_delta_base_cache_entry: use a more descriptive name
cache_or_unpack_entry: drop keep_cache parameter