git/nvim: Switch to using diffconflicts for resolving merge conflicts

While we dropped diffconflicts earlier and switch to vanilla vimdiff,
two way merges are definitely better than three way merges. 3 way merge
as in gina or vimdiff is extremely confusing except for may be the
simplest of merge conflicts.

conflict-marker and diffconflicts should let us handle all cases.

git/.gitconfig

@@ -18,15 +18,15 @@
 [diff]
 	colorMoved = default
 [merge]
-	tool = vimdiff
+	tool = diffconflicts
 	conflictstyle = diff3
 	ff = only
 [mergetool]
 	prompt = false
-[mergetool "vimdiff"]
-	cmd = nvim -d $BASE $LOCAL $REMOTE $MERGED -c '$wincmd w' -c 'wincmd J'
-	trustExitCode = true
 	keepBackup = false
+[mergetool "diffconflicts"]
+	cmd = 'nvim -c DiffConflicts "$MERGED" "$BASE" "$LOCAL" "$REMOTE"'
+	trustExitCode = true
 [rebase]
 	autoStash = true
 [commit]

nvim/.config/nvim/lua/plugins.lua

@@ -50,6 +50,7 @@ local init = function ()
   use 'rhysd/git-messenger.vim'
   use 'rhysd/conflict-marker.vim'
   use 'salcode/vim-interactive-rebase-reverse'
+  use 'whiteinge/diffconflicts'
   -- Boost vim command line mode
   use 'vim-utils/vim-husk'
   -- Rainbow Parentheses