Kdiff3 won't open with mergetool command

Question

I have conflicts, so I type:

git mergetool

I then get a message saying:

Hit return to start merge resolution tool

Answer 1

Yet another answer for future Googlers:

Actually, no external merge tool will start. An issue was filed in August 2015: https://jira.atlassian.com/browse/SRCTREEWIN-3543