Added a preliminary version of rename detection

JGit does not currently do rename detection during diffs. I added
a class that, given a TreeWalk to iterate over, can output a list
of DiffEntry's for that TreeWalk, taking into account renames. This
class only detects renames by SHA1's. More complex rename detection,
along the lines of what C Git does will be added later.

Change-Id: I93606ce15da70df6660651ec322ea50718dd7c04