git difftool --tool-help

$ git difftool --tool-help
'git difftool --tool=<tool>' may be set to one of the following:
		araxis
		meld
		vimdiff
			Opens vim with two vertical windows: LOCAL changes will be placed in the left
			window and REMOTE changes in the right one.
		vimdiff1
			Same as 'vimdiff'
		vimdiff2
			Same as 'vimdiff'
		vimdiff3
			Same as 'vimdiff'

	user-defined:
		default-difftool.cmd vim -f '+next' '+execute "DirDiff" argv(0) argv(1)' $LOCAL $REMOTE '+syntax off'
		vimdiff4.cmd vim -f -d -c "4wincmd w | wincmd J | tabnew | edit $LOCAL | vertical diffsplit $BASE | tabnew | edit $REMOTE | vertical diffsplit $BASE | 2tabprevious" "$LOCAL" "$BASE" "$REMOTE" "$MERGED"

The following tools are valid, but not currently available:
		bc
		bc3
		bc4
		codecompare
		deltawalker
		diffmerge
		diffuse
		ecmerge
		emerge
		examdiff
		guiffy
		gvimdiff
			Same as 'vimdiff' but opens 'gvim' instead (which uses a graphical toolkit for
			opening its own window)
		gvimdiff1
			Same as 'gvimdiff'
		gvimdiff2
			Same as 'gvimdiff'
		gvimdiff3
			Same as 'gvimdiff'
		kdiff3
		kompare
		nvimdiff
			Same as 'vimdiff' but opens 'neovim' instead (which is a fork of the original
			'vim' 'focused on extensibility and usability' according to their authors)
		nvimdiff1
			Same as 'nvimdiff'
		nvimdiff2
			Same as 'nvimdiff'
		nvimdiff3
			Same as 'nvimdiff'
		opendiff
		p4merge
		smerge
		tkdiff
		winmerge
		xxdiff

Some of the tools listed above only work in a windowed
environment. If run in a terminal-only session, they will fail.