1. Download SourceGear DiffMerge: http://sourcegear.com/diffmerge/index.html

2. Add the following to your global `.gitconfig` file:

        [diff]
            tool = DiffMerge
        [difftool "DiffMerge"]
            cmd = 'C:/Program Files/SourceGear/Common/DiffMerge/sgdm.exe' "$LOCAL" "$REMOTE"
        [merge]
            tool = DiffMerge
        [mergetool "DiffMerge"]
            cmd = 'C:/Program Files/SourceGear/Common/DiffMerge/sgdm.exe' -merge -result="$PWD/$MERGED" "$PWD/$LOCAL" "$PWD/$BASE" "$PWD/$REMOTE"
            trustExitCode = true
        [mergetool]
            keepBackup = false

After following these steps, you can run `git difftool` and `git mergetool` to open SourceGear DiffMerge from the Git Bash prompt.