revset: use `subset &` in `filelog`
This takes advantage of the `fullreposet` smartness.
revset #0: file(COPYING)
0) wall 3.179066 comb 3.180000 user 3.140000 sys 0.040000 (best of 3)
1) wall 2.723699 comb 2.730000 user 2.690000 sys 0.040000 (best of 4)