parsers: fix uninitialize variable warning The heads pointer is not initialized correctly if filter is false, causing both clang and gcc to issue a warning. Correctly initialize heads to NULL.

revset: use `subset &` in bare `p2()` This takes advantage of the `fullreposet` smartness with a nice speedup. It's a similar speedup to `p1()` when a merge is in progress (the non merge case is already lightning fast anyway.)

revset: use `subset &` in bare `p1()` This takes advantage of the `fullreposet` smartness and yields a nice speedup. revset #0: p1() 0) wall 0.003256 comb 0.010000 user 0.010000 sys 0.000000 (best of 527) 1) wall 0.000066 comb 0.000000 user 0.000000 sys 0.000000 (best of 23224)