Add blame link against parent tree in single-file diff view

This helps enable the workflow outlined in Issue 5, with one click for
each step:
-View blame of a file.
-Check full diff of a revision referred to by one line.
-See that was not the diff we were looking for, check blame at the
 parent revision.

Change-Id: Ib11706aaa5bab1a7ffe81f34568a72268e6393ea