When an interdiff is displayed in the diffviewer, we now show a history

of how the commits changed from the diffset to the interdiffset. The

implementation of diffing between two histories is very basic at the

moment. We currently only directly compare commit IDs as our metric of

whether or not two commits are equivalent. Once any commit IDs differ,

we assume the rest of the commits in the old history will be removed and

the remaining new commits are all added. This is because commit IDs

depend on prior commit IDs in most DVCS systems. Ideally we would use

use commit ancestor information (which is trivally obtainable from

Mercurial and less so from Git).

Ran JS tests.
Ran unit tests.