context: take advantage of `_descendantrev` in introrev if available

Before this changeset, `_descendantrev` was ignored and `introrev` could
return a "wrong" result. I was previously fine because there seems to be no
existing code using both `introrev` and `_descendantrev` at the same time.
However, we would like to change that.

mercurial/context.py

@@ -780,6 +780,11 b' class basefilectx(object):'
 
         if toprev is not None:
             return self._adjustlinkrev(toprev, inclusive=True)
+        elif r'_descendantrev' in attrs:
+            introrev = self._adjustlinkrev(self._descendantrev)
+            # be nice and cache the result of the computation
+            self._changeid = introrev
+            return introrev
         else:
             return self.linkrev()