Merge branch 'ew/many-alternate-optim'

Optimization for repositories with many alternate object store. * ew/many-alternate-optim: oidtree: a crit-bit tree for odb_loose_cache oidcpy_with_padding: constify `src' arg make object_directory.loose_objects_subdir_seen a bitmap avoid strlen via strbuf_addstr in link_alt_odb_entry speed up alt_odb_usable() with many alternates