route: scm: fix git and mercurial changes action If branch name has special character, "View revisions" link has route error. For example, Redmine Mercurial hgsubversion mirror has "../sandbox/rails-3.2" named branch. Git default branch of bare repository is defined at HEAD. In most cases, it is *master*. Mercurial GUI standard behavior is showing all revisions on top page. So, Redmine default branch is *nil*. Mercurial top page "View revisions" link has route error. git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@9617 e93f8b46-1217-0410-a6f0-8f06a7374b81