Fix bug with rendering of changing images in commits

When viewing the diff for a changed image file, two bugs occur:

- Both the 'before' and 'after' image link to the 'after' commit
- Visiting the  link for either image will cause a error

The first is caused by both image links referencing '@commit.id' rather
than '@commit.parent_id'.  The second is caused by the use of
'project_tree_path' which is used for creating links to directories in
the git file tree instead of 'project_blob_path' which links to files.