Fix out of range regression

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Fix out of range regression

From: Filip Brcic <[e-mail]>

The old revlog.py issued "index out of range" error when cloning the repository
Now I have reverted the parts of revlog.py to the old state when prev was
initialized as -1 and later assigned self.tip() only if that is possible.
Previously prev was always initialized as self.tip() and that is where the
out of range error was.

manifest hash: c94c9aee8b6d382ef52c3981f306a6e7e5f4c4d1
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mercurial/revlog.py

@@ -475,7 +475,7 b' class revlog:'
         t = r - 1
         node = nullid
 
-        base = -1
+        base = prev = -1
         start = end = measure = 0
         if r:
             start = self.start(self.base(t))
@@ -491,7 +491,6 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)