use difftool instead of mergetool

Author: moguls753

lib/thor/shell/basic.rb

@@ -377,21 +377,7 @@ def merge(destination, content) #:nodoc:
       end
 
       def merge_tool #:nodoc:
-        @merge_tool ||= ENV["THOR_MERGE"] || git_merge_tool
-      end
-
-      def git_merge_tool
-        tool = `git config merge.tool`.strip
-        return "" if tool.empty?
-
-        custom = `git config --get mergetool.#{tool}.cmd`.strip
-        return custom unless custom.empty?
-
-        return tool if system("command -v #{Shellwords.escape(tool)} > /dev/null 2>&1")
-
-        "git mergetool --no-prompt --tool=#{tool}"
-      rescue
-        ""
+        @merge_tool ||= ENV["THOR_MERGE"] || "git difftool --no-index"
       end
     end
   end