You might want to store a description of the changes, when you save a changed template.
You can use the revision log to store such information.
With the revision log you can:
The revision log does not store the complete, old version of a template. For that you might consider github or similar.
When you save a changed template in the designer, then you can choose to save with revision as below:
If you select Save with revision as marked above, then you can type in a description of the change in the pop up window:
The revision number automatically increase for each revision saved.
If you have chosen to save descriptions of template changes into the revision log, then you might want to display the contents.
You can see the revision log in your designer in two manners:
The revision log looks like this: