strip: fix revset usage (issue3604)

The `repair` code builds a giant revset query instead of using the "%lr" idiom.
It is inefficient and crash when the number of stripped changeset is too big.

This changeset replaces the bad code by a better revset usage.

mercurial/repair.py

@@ -111,9 +111,8 b' def strip(ui, repo, nodelist, backup="al'
         saverevs.difference_update(descendants)
     savebases = [cl.node(r) for r in saverevs]
     stripbases = [cl.node(r) for r in tostrip]
-    rset = ' or '.join([str(r) for r in tostrip])
-    newbmtarget = repo.revs('sort(heads(ancestors(%r) - (%r)), -rev)',
-                            rset, rset)
+    newbmtarget = repo.revs('sort(heads((::%ld) - (%ld)), -rev)',
+                            tostrip, tostrip)
     if newbmtarget:
         newbmtarget = newbmtarget[0]
     else: