If you are fine with the default console diff most of the time but only sometimes want to use an external tool for viewing a diff, you can use git difftool.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

E.g. viewing git diff with meld:

git difftool --tool=meld

For each file in the diff you will be asked if you want to view it using meld.