how to diff two branches

[weakish]

It seems that gl diff only accepts files as arguments.
So to diff two branches, I have to move HEAD back and forth?

gl switch b
gl branch -sh a
gl diff
gl branch -sh 2b5ec99 

If I do not want to memorize or copy-paste the revision hash,
I need to create a branch as a bookmark, then delete it afterwards.

gl switch b
gl switch -c tmp
gl branch -sh a
gl diff
gl branch -sh tmp
gl branch -d tmp