An example of .gitconfig for Notepad++ 7.5.9+ and WinMerge 2.16.4+.

.gitconfig

[core]
	editor = 'C:/Program Files/Notepad++/notepad++.exe' -multiInst -nosession
[mergetool]
	prompt = false
	keepBackup = false
	keepTemporaries = false
[merge]
	tool = winmerge
[mergetool "winmerge"]
	name = WinMerge
	trustExitCode = true
	# 3 way compare. Only the middle window can be modified. Saving changes updates the current branch
	cmd = "/c/Program\\ Files/WinMerge/WinMergeU.exe" -u -x -wr -wl -dl \"Local\" -dm \"Merge destination\" -dr \"Remote\" $LOCAL $BASE $REMOTE -o $MERGED
[diff]
	tool = winmerge
[difftool "winmerge"]
	name = WinMerge
	trustExitCode = true
	# Only the "Right" file can be modified. To change the file in the git project, make sure it is opened in the "Right" window
	# For example, having branches master and bob with the master branch being current, the correct command for file comparison with the possibility to
	# edit the file in the master branch is:
	# "git difftool bob README.md"	OR "git difftool bob master README.md"
	# For example, having branches master and bob with the master branch being current, the correct command for directory comparison with the possibility to edit
	# the files in the master branch is:
	# "git difftool -d bob"	OR "git difftool -d bob master"
	cmd = "/c/Program\\ Files/WinMerge/WinMergeU.exe" -u -x -wl -dl \"Left\" -dr \"Right\" $LOCAL $REMOTE