similar: do not look up and create filectx more than once
Benchmark with 50k added/removed files, on tmpfs:
$ hg addremove --dry-run --time -q
previous: real 16.070 secs (user 14.470+0.000 sys 1.580+0.000)
this patch: real 12.420 secs (user 11.120+0.000 sys 1.280+0.000)