This change adds the "post review" feature for mercurial

repositories. So it is possible to select changesets

with "New Review Request" page.
Remote repositories are supported by hgweb json

interface. Be aware that Mercurial 3.9 is required.
Local repositories are supported by json templates

interface via cmdline --template json.
See: https://selenic.com/repo/hg/help/hgweb
Bugs closed: 4032

HgWebClient:

  Used "hg serve" for remote repository!
HgLocalClient:

  Used local (directory) repository!
Switched between branches, scrolled list to get more changesets and created different reviews from changesets.