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
* Preserve current path when changing branch or revision
* Perform slightly more graceful error handling in the case of invalid repository URLs
* Allow branch names to contain periods
* Allow dashes in branch names
* Sort branches by name
* Adding tags dropdown
* Need to disable both branches and tags dropdowns before submitting revision form
* Support underscores in revision (branch/tag) names
* Making file history sensitive to current branch/tag/revision, adding common navigation to changes page
* Updated translation files to include labels for 'branch', 'tag', and 'view all revisions'
* Reenable fields after submit so they don't look disabled and don't stay disabled on browser back button
* Instead of dashes just use empty string for default dropdown value
* Individual entry views now sport the upgraded revision navigation
* Don't display dropdowns with no entries
* Consider all revisions when doing initial load
* Fixed bug grabbing changesets.  Thanks to Bernhard Furtmueller for catching.
* Always check the entire log to find new revisions, rather than trying to go forward from the latest known one
* Added some cleverness to avoid selecting the whole changesets table any time someone views the repository root
* File copies and renames being detected properly
* Return gracefully if no revisions are found in the git log
* Applied patch from Babar Le Lapin to improve Windows compatibility

git-svn-id: https://svn.redmine.org/redmine/trunk@2840 e93f8b46-1217-0410-a6f0-8f06a7374b81