revlog: move delta check to it's own function

This moves the delta vs fulltext comparison to its own function. This will allow
us to reuse the function in future patches for more efficient delta choices. As
a side effect, this will also allow extensions to modify our delta criteria.

mercurial/revlog.py

@@ -1233,6 +1233,25 b' class revlog(object):'
             return ('u', text)
         return ("", bin)
 
+    def _isgooddelta(self, d, textlen):
+        """Returns True if the given delta is good. Good means that it is within
+        the disk span, disk size, and chain length bounds that we know to be
+        performant."""
+        if d is None:
+            return False
+
+        # - 'dist' is the distance from the base revision -- bounding it limits
+        #   the amount of I/O we need to do.
+        # - 'compresseddeltalen' is the sum of the total size of deltas we need
+        #   to apply -- bounding it limits the amount of CPU we consume.
+        dist, l, data, base, chainbase, chainlen, compresseddeltalen = d
+        if (dist > textlen * 4 or l > textlen or
+            compresseddeltalen > textlen * 2 or
+            (self._maxchainlen and chainlen > self._maxchainlen)):
+            return False
+
+        return True
+
     def _addrevision(self, node, text, transaction, link, p1, p2, flags,
                      cachedelta, ifh, dfh):
         """internal function to add revisions to the log
@@ -1334,13 +1353,7 b' class revlog(object):'
         else:
             textlen = len(text)
 
-        # - 'dist' is the distance from the base revision -- bounding it limits
-        #   the amount of I/O we need to do.
-        # - 'compresseddeltalen' is the sum of the total size of deltas we need
-        #   to apply -- bounding it limits the amount of CPU we consume.
-        if (d is None or dist > textlen * 4 or l > textlen or
-            compresseddeltalen > textlen * 2 or
-            (self._maxchainlen and chainlen > self._maxchainlen)):
+        if not self._isgooddelta(d, textlen):
             text = buildtext()
             data = self.compress(text)
             l = len(data[1]) + len(data[0])