filemerge: refactoring of 'filemerge()'
current 'filemerge.filemerge()' implementation is verfy complicated.
- it is not easy to add new internal merge tools
(only by patching on 'filemerge()', or replacing it completely)
- cleanup of temporary files is unsatisfactory
('internal:dump' does not, in fact)
this is patch for refactoring of 'filemerge()' to isolate each
internal merge tool implementations from 'filemerge()', and clean up
common part in it.