Using WinMerge as the git Diff/Merge Tool on Windows 64bit

.gitconfig

[mergetool]
    prompt = false
    keepBackup = false
    keepTemporaries = false

[merge]
    tool = winmerge

[mergetool "winmerge"]
    name = WinMerge
    trustExitCode = true
    cmd = "/c/Program\\ Files\\ \\(x86\\)/WinMerge/WinMergeU.exe" -u -e -wl -wr $LOCAL $BASE $REMOTE -o $MERGED

[diff]
    tool = winmerge

[difftool "winmerge"]
    name = WinMerge
    trustExitCode = true
    cmd = "/c/Program\\ Files\\ \\(x86\\)/WinMerge/WinMergeU.exe" -u -e $LOCAL $REMOTE

This version is modified to put local/base/remote in the proper order. See also this illustration of how vimdiff does it.