MeldDiffTool

input: old_dir, new_dir, suffix, [failed_files]

for each failed file open meld with 3 tabs,
1.) two windows showing how the file changed upstream
2.) two windows showing how the patch changed the file in old_dir
3.) three windows
    left) new upstream version of the file
    right) old version with patch applied
    middle) (??) show conflict, you need to select version either
            from left window (new upstream) or
            from right window (old version with patch applied)

see http://stackoverflow.com/questions/11133290/git-merging-using-meld
Merge remote-tracking branch 'upstream/master'