upgrade: move most of revlog.clone method into a _clone method

The content of the clone method now focus on parameters validation and
processing. The `_clone` method focus on the actual cloning logic.

Splitting the method out save some indentation and clarify each method code
since it a focussed on one goal.

Differential Revision: https://phab.mercurial-scm.org/D6899

mercurial/revlog.py

@@ -2405,6 +2405,17 b' class revlog(object):'
 
             destrevlog._deltabothparents = forcedeltabothparents or oldamd
 
+            self._clone(tr, destrevlog, addrevisioncb, deltareuse,
+                        forcedeltabothparents)
+
+        finally:
+            destrevlog._lazydelta = oldlazydelta
+            destrevlog._lazydeltabase = oldlazydeltabase
+            destrevlog._deltabothparents = oldamd
+
+    def _clone(self, tr, destrevlog, addrevisioncb, deltareuse,
+               forcedeltabothparents):
+        """perform the core duty of `revlog.clone` after parameter processing"""
             deltacomputer = deltautil.deltacomputer(destrevlog)
             index = self.index
             for rev in self:
@@ -2422,8 +2433,7 b' class revlog(object):'
                 # the revlog chunk is a delta.
                 cachedelta = None
                 rawtext = None
-                if (deltareuse != self.DELTAREUSEFULLADD
-                        and destrevlog._lazydelta):
+            if (deltareuse != self.DELTAREUSEFULLADD and destrevlog._lazydelta):
                     dp = self.deltaparent(rev)
                     if dp != nullrev:
                         cachedelta = (dp, bytes(self._chunk(rev)))
@@ -2454,10 +2464,6 b' class revlog(object):'
 
                 if addrevisioncb:
                     addrevisioncb(self, rev, node)
-        finally:
-            destrevlog._lazydelta = oldlazydelta
-            destrevlog._lazydeltabase = oldlazydeltabase
-            destrevlog._deltabothparents = oldamd
 
     def censorrevision(self, tr, censornode, tombstone=b''):
         if (self.version & 0xFFFF) == REVLOGV0: