changeset: fix XSS vulnerability in parent-child navigation

The 'Parent Rev.' - 'Child Rev.' links on changesets and in the file browser
normally immediately jump to the correct revision upon click. But, if there
are multiple candidates, e.g. two children of a commit, then a list of
revisions is shown as hyperlinks instead.

These hyperlinks have a 'title' attribute containing the full commit message
of the corresponding commit. When this commit message contains characters
special to HTML, like ", >, etc. they were added literally to the HTML code.

This can lead to a cross-site scripting (XSS) vulnerability when an attacker
has write access to a repository. They could craft a special commit message
that would introduce HTML and/or JavaScript code when the commit is listed
in such 'parent-child' navigation links.

Escape the commit message before using it further.