Merge branch 'jh/add-index-entry-optim'

Merge branch 'jh/add-index-entry-optim'

"git checkout" that handles a lot of paths has been optimized by
reducing the number of unnecessary checks of paths in the
has_dir_name() function.

* jh/add-index-entry-optim:
  read-cache: speed up has_dir_name (part 2)
  read-cache: speed up has_dir_name (part 1)
  read-cache: speed up add_index_entry during checkout
  p0006-read-tree-checkout: perf test to time read-tree
  read-cache: add strcmp_offset function