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

diff --cc Makefile
Simple merge

diff --cc dir.c
Simple merge

diff --cc dir.h
Simple merge

diff --cc object-file.c
Simple merge

diff --cc t/helper/test-tool.c
Simple merge

diff --cc t/helper/test-tool.h
Simple merge