filter: add a comment so that people do not forget to update subsettable

Changeset 175c6fd8cacc moved `subsettable` from `mercurial/repoview.py` to
`mercurial/branchmap.py`. This mean that `filtertable` and `subsettable` are no
longer next to each other. So we add a comment to remind people to update both.

mercurial/repoview.py

@@ -94,6 +94,11 b' def computeimpactable(repo):'
     return frozenset(xrange(firstmutable, len(cl)))
 
 # function to compute filtered set
+#
+# When addding a new filter you MUST update the table at:
+#     mercurial.branchmap.subsettable
+# Otherwise your filter will have to recompute all its branches cache
+# from scratch (very slow).
 filtertable = {'visible': computehidden,
                'served': computeunserved,
                'immutable':  computemutable,