nixpkgs/elvish: don't set difft as GIT_EXTERNAL_DIFF

This messes up other tools where we'd normally expect a regular git diff
output. Move this to difftool instead. This change does require setting
diff.tool to difft via something like ~/.gitconfig.

.config/git/config

@@ -16,6 +16,10 @@
 	ui = true
 [diff]
 	algorithm = patience
+[difftool]
+	prompt = false
+[difftool "difft"]
+	cmd = difft "$LOCAL" "$REMOTE"
 [push]
 	default = simple
 [alias]

.config/nixpkgs/elvish/lib/config.elv

@@ -12,10 +12,6 @@ if (has-external yadm) {
   edit:add-var yadm~ { |@a| git --git-dir=~/.dotfiles/ --work-tree=~ $@a }
 }
 
-if (has-external difft) {
-  set-env GIT_EXTERNAL_DIFF difft
-}
-
 if (has-external delta) {
   set-env GIT_PAGER delta
 }