base: escape branch/tag/bookmark names in 'Switch To' menu to prevent XSS

On repository pages, the 'Switch To' did not escape branches correctly.

This means that if an attacker is able to push a branch/tag/bookmark
containing HTML/JavaScript in its name, then that code would be evaluated.
This is a cross-site scripting (XSS) vulnerability.

Fix the problem by correctly escaping the branch/tag/bookmarks with
.html_escape() .