Using kdiff3 as diff tool, user has to specify name of output file after merge

[claremacrae]

Reported by @bjosv

During the experiments with ApprovalTests.cpp and kdiff3 we have seen that there is a need to click in the menu and to enter a filename. This can be avoided by giving the information when starting kdiff3 instead, which correlates with how other diff-tools are started in ApprovalTests.cpp.