dirstate: test normalize is truthy instead of using a no-op lambda
hg perfstatus -u on a working directory with 170,000 files, without this
change:
! wall 1.869404 comb 1.850000 user 1.170000 sys 0.680000 (best of 6)
With this change:
! wall 1.839561 comb 1.830000 user 1.120000 sys 0.710000 (best of 6)
....................
----------------------------------------------------------------------
Ran 20 tests in 0.000s
OK