It turned out that tortoiseGit's diff and merge tools were separate from git.
I succeeded in configuring tortoiseGit with LVMerge and LVDiff.
Although I haven't really succeeded in configuring git with LVMerge and LVDiff, I think it would be better to configure difftool and mergetool than low level diff and merge drivers for LabVIEW.
There are two reasons.
1) If I set up a low level diff driver for LVDiff, git extensions would launch LVDiff whenever I select a file in git extension's commit window, which is very annoying.
2) If I set up a low level merge driver for LVDiff, every "git merge" operation might launch LVMerge for every file that needs merging, which again can be annoying.
Since neither tortoiseGit nor git extensions doesn't use "git difftool" or "git mergetool", setting difftool and mergetool along with tortoiseGit for LabVIEW would make it easier to use both tortoiseGit and git extensions.