Just to extend the @Joseph's answer:
After applying these commands your global .gitconfig
file will have the following lines (to speed up the process you can just copy them in the file):
[merge]
tool = kdiff3
[mergetool "kdiff3"]
path = C:/Program Files/KDiff3/kdiff3.exe
trustExitCode = false
[diff]
guitool = kdiff3
[difftool "kdiff3"]
path = C:/Program Files/KDiff3/kdiff3.exe
trustExitCode = false