How do I do a diff between 2 tags without checking out both tags?
Go to Revision Graph.
CTRL-click on the 2 revisions where the tags were marked.
Right-click, select "Compare Revisions"
Start the Repository Browser, navigate to the newer tag and use the context menu to show the log. Disable the checkbox 'stop on copy'. Select both revisions to compare and use the context menu again to actually do the compare.
You just want to diff two folders right?
Right click on the 1st folder, Select "Mark for comparison"
Right click on the 2nd folder, select "Show differences as unified diff"
How do I do a diff between 2 tags without checking out both tags?
This can be interpreted as if checking out one tag is fine:
Check out one tag. Right-click on the folders while pressing SHIFT, select "Diff with URL", and enter the other tag's URL.
For a summarized diff, start the repo browser, highlight both the tags you want to compare, right click and choose "compare revisions". This lists the files/folders changes.
If you need the exact file-wise line differences in a text file, you can opt for "show differences as unified diff"