I have conflicts, so I type:
git mergetool
I then get a message saying:
Hit return to start merge resolution tool
>
Again, for future Googlers:
As of version 2.48.02 (29 November 2014), Git Extensions started distributing the 64-bit version of kdiff3. (See https://github.com/gitextensions/gitextensions/blob/master/GitUI/Resources/ChangeLog.md#version-24802-29-november-2014.)
So if you're running a 32-bit OS and had the Git Extensions installer install kdiff3, your kdiff3 won't even run by itself. The solution is to download the 32-bit version (http://sourceforge.net/projects/kdiff3/files/kdiff3/) and reinstall. I didn't even need to uninstall the 64-bit version first, as the installer simply overwrote the previous install.