compare: correct display of special branch names in initial placeholder

When a branch name contains special characters like '<' or '>', and a
'compare' operation is performed with such branch as one of the two compare
sides, then the special branch name will be part of the URL, e.g.

http://localhost:5000/myrepo/compare/branch@master...branch@%3Cscript%3Eblabla%3C/script%3E?other_repo=myrepo

The encoded branch name is then used at page load as placeholders for the
branch selection dropdowns. But, the special characters, were escaped too
much, causing '<' to become &lt; in the display of the dropdown.

The placeholder was escaped via the default mako escape filter, before being
passed to make_revision_dropdown, thus too early. We want the raw value.
h.js() (copied from the default branch) gives us that, while still
formatting and escaping the string so it is safe inside the script tag.