filemerge: allow the formatting of three labels instead of two When a third label is provided (to included the base content) it is properly processed as the two others. Nothing changes if only two labels are provided.

filemerge: drop extra white space There should be no white space around the brace.

simplemerge: support three labels when merging If a third label is provided it will be used for the "base" content: <<<<<<< local content from local ||||||| base former common ======= other conflicting >>>>>>> other