Print old and new git revisions in `ob deploy update`

[mightybyte]

Every time I do an ob deploy update I find myself wanting to know what the previous git revision was, whether it changed, and if so what the new revision is. Seems like it should be pretty easy to add this. Just thought I'd throw it here to not let the thought get lost.

[alexfmpe]

Personally I just use git diff and see what changed in the src thunk. Las time I checked ob deploy update basically did ob thunk update src