revrange: build spanset from x:y range This slightly improves the performance in the optimal case: % hg log -R mozilla-central -r0:tip -l1 --time (before) time: real 0.050 secs (user 0.040+0.000 sys 0.010+0.000) (after) time: real 0.020 secs (user 0.000+0.000 sys 0.010+0.000)