how to compare branches?

From: Steve Kargl <sgk_at_troutmask.apl.washington.edu>
Date: Mon, 24 Jul 2023 19:37:37 UTC 
It has been brought to my attention that a few fixes to libm
in main have not been merged to at least the 13 branch.  To
check on the level of missing changes, I would to compare ther
lib/msun on main to lib/msun on stable/13.  What is the git
command needed for such a task?  My attempts of modifying results
from a google search of "git diff between branches" has been
entertaining but fruitless.

-- 
Steve