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)

mercurial/scmutil.py

@@ -737,8 +737,10 b' def revrange(repo, revs):'
                 end = revfix(repo, end, len(repo) - 1)
                 if end == nullrev and start < 0:
                     start = nullrev
-                rangeiter = repo.changelog.revs(start, end)
-                l = revset.baseset(rangeiter)
+                if start < end:
+                    l = revset.spanset(repo, start, end + 1)
+                else:
+                    l = revset.spanset(repo, start, end - 1)
                 subsets.append(l)
                 continue
             elif spec and spec in repo: # single unquoted rev