templates/summary: escape branch/tag/bookmark names in 'Download as zip' links to prevent XSS

On a repository summary page, in the 'Download' section where you can
download an archive of the repository at a given revision, the branch/tag
names were not correctly escaped.

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.

Reported by Bob Hogg <wombat@rwhogg.site> (thanks!).