* 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