revlog: avoid opening and closing the file for each cloned revision

The previous code was flushing files after each new revision, slowing things
down.

For exemple, with this change, the evolve repository can run
`hg debugupgraderepo --run --optimize re-delta-parent` in about 3.4s instead of
4.5 seconds.

mercurial/revlog.py

@@ -3461,7 +3461,7 b' class revlog:'
             )
             destrevlog.delta_config.delta_both_parents = delta_both_parents
 
-            with self.reading():
+            with self.reading(), destrevlog._writing(tr):
                 self._clone(
                     tr,
                     destrevlog,
@@ -3552,19 +3552,18 b' class revlog:'
                     )
                     flags = flags | new_flags[0] & ~new_flags[1]
 
-                with destrevlog._writing(tr):
-                    destrevlog._addrevision(
-                        node,
-                        rawtext,
-                        tr,
-                        linkrev,
-                        p1,
-                        p2,
-                        flags,
-                        cachedelta,
-                        deltacomputer=deltacomputer,
-                        sidedata=sidedata,
-                    )
+                destrevlog._addrevision(
+                    node,
+                    rawtext,
+                    tr,
+                    linkrev,
+                    p1,
+                    p2,
+                    flags,
+                    cachedelta,
+                    deltacomputer=deltacomputer,
+                    sidedata=sidedata,
+                )
 
             if addrevisioncb:
                 addrevisioncb(self, rev, node)