filemerge: create detail of internal merge tools from documentation string this patch introduces 'internaltoolsmarker' which creates detail of each internal merge tools from documentation string for 'hg help merge-tools'.

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.