revlog: document high frequency of code execution

Recording my notes while working on performance optimization.

mercurial/revlog.py

@@ -1471,6 +1471,10 b' class revlog(object):'
         # should we try to build a delta?
         if prev != nullrev:
             tested = set()
+            # This condition is true most of the time when processing
+            # changegroup data into a generaldelta repo. The only time it
+            # isn't true is if this is the first revision in a delta chain
+            # or if ``format.generaldelta=true`` disabled ``lazydeltabase``.
             if cachedelta and self._generaldelta and self._lazydeltabase:
                 # Assume what we received from the server is a good choice
                 # build delta will reuse the cache