perf: add perffncacheencode
Examples (all done with somewhat dated clones I found on my disk):
Netbeans (~120k entries in fncache):
$ hg perffncacheencode
! wall 4.338000 comb 4.336828 user 4.336828 sys 0.000000 (best of 3)
Openoffice (~77k entries in fncache)):
$ hg perffncacheencode
! wall 1.533000 comb 1.528810 user 1.528810 sys 0.000000 (best of 7)
Xen (~10k entries in fncache):
$ hg perffncacheencode
! wall 0.198000 comb 0.187201 user 0.187201 sys 0.000000 (best of 51)
Done on Windows 7 x64.