Warn contributors from doing work that will no be merged

From time to time contributors spend time doing work that will not be
accepted as it duplicates functionality that is already provided with
the mapping files. Try to dissuade them from doing that by explaining
the reasons in the comment.

hg-fast-export.py

@@ -153,9 +153,13 @@ def sanitize_name(name,what="branch", mapping={}):
   # modifying names which previously were not touched it will break
   # preexisting setups which are doing incremental imports.
   #
-  # Use the -B and -T options to mangle branch and tag names
-  # instead. If you have a source repository where this is too much
-  # work to do manually, write a tool that does it for you.
+  # Fast-export tries to not inflict arbitrary naming policy on the
+  # user, instead it aims to provide mechanisms allowing the user to
+  # apply their own policy. Therefore do not add a transform which can
+  # already be implemented with the -B and -T options to mangle branch
+  # and tag names. If you have a source repository where this is too
+  # much work to do manually, write a tool that does it for you.
+  #
 
   def dot(name):
     if not name: return name