Project commit count for default branch

Currently, the commit count displayed on a project's home page shows the total number of commits on the master branch. This can lead to situations in which a project may have no commits reported or very few if the project workflow does not involve the use of a branch named "master". Since number of commits can be an indicator of project health, showing 0 commits could make it harder to encourage adoption of a project.

This MR alters the repository model's `commit_count` method to return the count for the default branch instead of master.

See issue #299