repair: speed up stripping of many roots

repair.strip() expects a set of root revisions to strip. It then
builds the full set of descedants by walking the descandants of
each. It is rare that more than a few roots get passed in, but if that
happens, it will wastefully walk the changelog for each root. So let's
just walk it once.

I noticed this because the narrowhg extension was passing not only
roots, but all the commits to strip. When there were tens of thousands
of commits to strip, this resulted in quadratic behavior with that
extension.

mercurial/repair.py

@@ -99,9 +99,9 b' def strip(ui, repo, nodelist, backup=Tru'
     # (head = revision in the set that has no descendant in the set;
     #  base = revision in the set that has no ancestor in the set)
     tostrip = set(striplist)
-    for rev in striplist:
-        for desc in cl.descendants([rev]):
-            tostrip.add(desc)
+    for r in cl.revs(start=striprev + 1):
+        if any(p in tostrip for p in cl.parentrevs(r)):
+            tostrip.add(r)
 
     files = _collectfiles(repo, striprev)
     saverevs = _collectbrokencsets(repo, files, striprev)