store: optimize fncache._load a bit by dirdecoding the contents in one go
For a netbeans clone on Windows 7 x64:
Before:
$ hg perffncacheload
! wall 0.124000 comb 0.124801 user 0.124801 sys 0.000000 (best of 76)
After:
$ hg perffncacheload
! wall 0.096000 comb 0.093601 user 0.078001 sys 0.015600 (best of 97)