repair: use `first` instead of direct indexing

This makes it compatible with all smartset classes.

mercurial/hg.py

@@ -24,7 +24,14 b' def addbranchrevs(lrepo, other, branches'
     peer = other.peer() # a courtesy to callers using a localrepo for other
     hashbranch, branches = branches
     if not hashbranch and not branches:
-        return revs or None, revs and revs[0] or None
+        x = revs or None
+        if util.safehasattr(revs, 'first'):
+            y =  revs.first()
+        elif revs:
+            y = revs[0]
+        else:
+            y = None
+        return x, y
     revs = revs and list(revs) or []
     if not peer.capable('branchmap'):
         if branches:

mercurial/repair.py

@@ -99,7 +99,7 b' def strip(ui, repo, nodelist, backup=Tru'
     # is much faster
     newbmtarget = repo.revs('max(parents(%ld) - (%ld))', tostrip, tostrip)
     if newbmtarget:
-        newbmtarget = repo[newbmtarget[0]].node()
+        newbmtarget = repo[newbmtarget.first()].node()
     else:
         newbmtarget = '.'