After installing it https://download.gnome.org/binaries/win32/meld/ I had to tell Git to use Meld:
git config --global merge.tool meld
git config --global diff.tool meld
The last command tells Git where it can find Meld and depends on whether you are using Cygwin or Git for Windows:
Git for Windows:
git config --global mergetool.meld.path 'C:\Program Files (x86)\Meld\Meld.exe'
Cygwin:
git config --global mergetool.meld.path '/cygdrive/c/Program Files (x86)/Meld/Meld.exe'
And that seems to work. Both merging and diffing with "git difftool" or "git mergetool"
Based on https://gist.github.com/kjlubick/5a49a3ae5f39ae359997
Is there a 64 bit version of meld for Windows?