revlog: move file writing to a separate function

Moves the code that actually writes to a file to a separate function in
revlog.py. This allows extensions to intercept and use the data being written to
disk. For example, an extension might want to replicate these writes elsewhere.

When cloning the Mercurial repo on /dev/shm with --pull, I see about a 0.3% perf change.
It goes from 28.2 to 28.3 seconds.

mercurial/revlog.py

@@ -1206,6 +1206,15 b' class revlog(object):'
         self.nodemap[node] = curr
 
         entry = self._io.packentry(e, self.node, self.version, curr)
+        self._writeentry(transaction, ifh, dfh, entry, data, link, offset)
+
+        if type(text) == str: # only accept immutable objects
+            self._cache = (node, curr, text)
+        self._basecache = (curr, chainbase)
+        return node
+
+    def _writeentry(self, transaction, ifh, dfh, entry, data, link, offset):
+        curr = len(self) - 1
         if not self._inline:
             transaction.add(self.datafile, offset)
             transaction.add(self.indexfile, curr * len(entry))
@@ -1222,11 +1231,6 b' class revlog(object):'
             ifh.write(data[1])
             self.checkinlinesize(transaction, ifh)
 
-        if type(text) == str: # only accept immutable objects
-            self._cache = (node, curr, text)
-        self._basecache = (curr, chainbase)
-        return node
-
     def addgroup(self, bundle, linkmapper, transaction):
         """
         add a delta group