Sometimes it is needed to use some graphical applications for viewing git diff or merge. Git allows to specify it.

The simplest way is to use in command line:

git difftool --tool=meld

or for merging:

git mergetool --tool=meld

where meld is a command for your application. In my case it is meld.

If you don't want every time specify it you have to configure globally:

git config --global diff.tool meld

or

git config --global merge.tool meld

Now it is possible to run them executing only git difftool or git mergetool in you repository directory.