revlog: split and document good delta conditional

The logic is still identical, but having each conditional on its own helps to
document them and will help to edit them in the future.

mercurial/revlog.py

@@ -2473,9 +2473,30 b' class revlog(object):'
             # certain size. Be also apply this tradeoff here and relax span
             # constraint for small enought content.
             maxdist = self._srmingapsize
-        if (distance > maxdist or deltainfo.deltalen > textlen or
-            deltainfo.compresseddeltalen > textlen * 2 or
-            (self._maxchainlen and deltainfo.chainlen > self._maxchainlen)):
+
+        # Bad delta from read span:
+        #
+        #   If the span of data read is larger than the maximum allowed.
+        if maxdist < distance:
+            return False
+
+        # Bad delta from new delta size:
+        #
+        #   If the delta size is larger than the target text, storing the
+        #   delta will be inefficient.
+        if textlen < deltainfo.deltalen:
+            return False
+
+        # Bad delta from cumulated payload size:
+        #
+        #   If the sum of delta get larger than K * target text length.
+        if textlen * 2  < deltainfo.compresseddeltalen:
+            return False
+
+        # Bad delta from chain length:
+        #
+        #   If the number of delta in the chain gets too high.
+        if self._maxchainlen and  self._maxchainlen < deltainfo.chainlen:
             return False
 
         return True