We only handle branch selection for repo home and commits pages, so the
redirection URL is based on PageIsCommits

I've mostly duplicated the dropdown code from repo/home.tmpl, which
basically only required a change to the URL. This could probably be
broken out into something more modular.

This commit improves templates readability, since all of them use consistent
indent with all template command blocks indented too.

1. Indents both HTML containers such as <div>, <p> and Go HTML template blocks
   such as {{if}} {{with}}

2. Cleans all trailing white-space

3. Adds trailing last line-break to each file

Merges repo/sidebar.tmpl with repo/header.tmpl and makes every repo page use
middleware.RepoRef() necessary to display information on this menu.

This is more inline with way GitHub looks like and feels much more natural and
in style with rest of the interface.