Fix corruption resulting from skipping parts of a revision group

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Fix corruption resulting from skipping parts of a revision group

We were occassionally losing track of what revision a delta applied to
when we skipped over deltas we already had and applying the delta
against the wrong base. This could result in coredumps from mpatch,
consistency errors, or failed verify.

manifest hash: fcf20a8abfd81f08fae2398136b2ed66216b2083
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mercurial/revlog.py

@@ -475,8 +475,8 b' class revlog:'
         t = r - 1
         node = nullid
 
-        base = prev = -1
-        start = end = 0
+        base = -1
+        start = end = measure = 0
         if r:
             start = self.start(self.base(t))
             end = self.end(t)
@@ -491,6 +491,7 b' class revlog:'
 
         # loop through our set of deltas
         chain = None
+        prev = self.tip()
         for chunk in revs:
             node, p1, p2, cs = struct.unpack("20s20s20s20s", chunk[:80])
             link = linkmapper(cs)
@@ -498,6 +499,7 b' class revlog:'
                 # this can happen if two branches make the same change
                 if unique:
                     raise "already have %s" % hex(node[:4])
+                chain = node
                 continue
             delta = chunk[80:]