Base concatenation markers on `mergedname'

This fixes the following bug:

  You register the same file name with the `concatenate'-enabled
  `filter' method, more than once with different destination
  directories. What happens is this:

    - The first version of the file is deployed correctly.

    - All other versions of the file are *NOT* truncated with the
      first part of the concatenation procedure. That means that the
      destination file grows each time one of the source files changes
      and the file is re-deployed, _appending_ to the existing file.

Since this keeps track of the "first-part" information based on the
final fully-qualified file name, this should not happen anymore.