Add 4th argument support for 4-way git merge

[yangit]

According to this comment here http://blog.wuwon.id.au/2010/09/painless-merge-conflict-resolution-in.html

Meld has hidden 3-way merge feature activated by passing the 4th parameter: 
[merge] tool = mymeld 
conflictstyle = diff3 
[mergetool "mymeld"] cmd = meld $LOCAL $BASE $REMOTE $MERGED 

You can use $MERGED state where git was able to merge and use last common base where auto merge was not able to merge. That way manual merge is much more easier.

Is 4th argument not available under mac only or is it not working on original meld as well?

[Wizek]

I'm also interested in being able to do 4-way merge with meld.

[moustachedelait]

I tried to get this to work by setting my git config mergetool cmd to: open -W -a Meld --args --auto-merge \"$LOCAL\" \"$BASE\" \"$REMOTE\" --output=\"$MERGED\" && open -W -a Meld --args \"$BASE\" \"$LOCAL\" && open -W -a Meld --args \"$BASE\" \"$REMOTE\"

But that crashes meld, and I can't even force quit it