What version are you running?
2.0.20 / 2.5
Describe the enhancement and the motivation for it.
I like to request the support for "post review" of Mercurial repositories.
Please provide any additional information below.
Mercurial (supported hgweb) provides a REST-JSON-API to get the needed information. I use selenic.com as an example as it provides a hgweb interface that "hg.py" of reviewboard already use.

function get_branches:

Example: https://selenic.com/repo/hg/branches?style=json

function get_commits:

Example: https://selenic.com/repo/hg/shortlog/default?revcount=180&style=json
"default" needs to be the branch or an explicit revision and "revcount" is optional to have a paginator.

function get_change:

Example: https://selenic.com/repo/hg/raw-rev/ef6f98473a48