Added branch and tag support to the git repository viewer. (#1406)

Many thanks to Adam Soltys and everyone else who tested this patch.

* Updated git test repository so it has a branch with some differences from the master branch
* Moved redmine diff class into a module so as not to clash with diff-lcs gem which is required by grit
* Find changesets from all branches, not just master
* Got revision browsing working
* Got file actions working properly
* Allow browsing by short form of commit identifier
* Added a method to retrieve repository branches
* Allow browsing by branch names as well as commit numbers
* Handle the case where a git repository has no master branch
* Expand revision box and handle finding revisions by first 8 characters
* Added branches dropdown to repository show page
* Combined repository browse and show into a single action.  Moved branch/revision navigation into a partial.
* Renamed partial navigation -> breadcrumbs
* Made it so latest revisions list uses branch and path context
* ...