Add more menuitems on repo pages

In an attempt to get better usability, a set of 'semistatic' menuitems
are added to the page header on all pages except the repository index.

The menuitems (summary, log, files, commit and diff) honours the current
branch and revision. To switch the current branch one can use the branch
links on the summary page.

The backlink to the repository index page is now available by clicking
the static page heading.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>