Fix gitlab network graph without master branch

gitlab-graph-no-master

Gitlab uses Grit for the network graph, and Grit relies on the master branch for this functionality.  Without a master branch the graph shows up blank, however, you can get it to show the graph for your default branch only (instead of all branches as it would normally show) by changing the following line in lib/graph_commit.rb:

    commits = Grit::Commit.find_all(@repo, nil, {:max_count => 650})

With this:

    commits = Grit::Commit.find_all(@repo, project.root_ref, {:max_count => 650})