## set your difftool once:
git config --global diff.tool meld
## then use
git difftool -d master other_branch
## to see differences
See http://stackoverflow.com/a/2006241/2574238
git checkout master
git difftool -d other_branch
Now one can edit the right window. Saved changes are permanent. (see http://stackoverflow.com/a/22535996/2574238)
git mergetool