There is a repository located at:
How do I view list of changes (as commits and messages) between branches 2.8 and 3.0?
I tried
$ hg log -r "2.8:3.0" --template '{node|short} {desc|strip|firstline}\n'
e1e5cf5700d0 Increase version number
faa26bc2e908 Added tag 2.8.12 for changeset a81a53304344
a81a53304344 Prepare release 2.8.12
b57e24462eb1 Increase version number
But it lists not all changes made between 2.8 and 3.0
Log of changes between branches?! How you'll imagine this? Branches are independent (mutable) trees with (possible) common ancestor in the past
Revset with ":" range produced, as expected
When you use branch-name as revision-id, branch's tip used as node, and before any commits into any of affected by revset branches, you'll get only this result
While I can't find any reasonable backend in you query, I have to ask "Which was your real business-task?"
Add-on
Log with hardcoded revset for "revisions between two last tags in branches 2.8 and 3.0" will be
hg log -r "last(tag('re:^2.8')):last(tag('re:^3.0'))"
and revset can be easy converted into the parametrized
[revsetalias]
likecbt($1, $2) = last(tag('re:^$1')):last(tag('re:^$2'))
and log can be shortened to
hg log -r "cbt(2.8, 3.0)"
and used for any pair of branches