replay: Filter externals to copy in copyfrom operations
When a directory is copied from another one in a previous revision, externals must be copied only if they have been defined in a revision greater or equal to the revision the directory is copied from.
So store the revision number an external is defined and use it to filter externals when performing copyfrom operations.
This fixes the loading of https://svn.code.sf.net/p/dlvhex/code (and likely other repos) in SWH-LOADER-SVN-C4