Merge branch 'kb/checkout-optim'

Merge branch 'kb/checkout-optim'

* kb/checkout-optim:
  Revert "lstat_cache(): print a warning if doing ping-pong between cache types"
  checkout bugfix: use stat.mtime instead of stat.ctime in two places
  Makefile: Set compiler switch for USE_NSEC
  Create USE_ST_TIMESPEC and turn it on for Darwin
  Not all systems use st_[cm]tim field for ns resolution file timestamp
  Record ns-timestamps if possible, but do not use it without USE_NSEC
  write_index(): update index_state->timestamp after flushing to disk
  verify_uptodate(): add ce_uptodate(ce) test
  make USE_NSEC work as expected
  fix compile error when USE_NSEC is defined
  check_updates(): effective removal of cache entries marked CE_REMOVE
  lstat_cache(): print a warning if doing ping-pong between cache types
  show_patch_diff(): remove a call to fstat()
  write_entry(): use fstat() instead of lstat() when file is open
  write_entry(): cleanup of some duplicated code
  create_directories(): remove some memcpy() and strchr() calls
  unlink_entry(): introduce schedule_dir_for_removal()
  lstat_cache(): swap func(length, string) into func(string, length)
  lstat_cache(): generalise longest_match_lstat_cache()
  lstat_cache(): small cleanup and optimisation