Also log the cwd when git commands fail

Change-Id: I6c1b558535162d7cf87ba5a7b56adc00739e3014
Clarify to git that we're after the diff of a revision

The -- separates revisions from files.

Change-Id: Ib082964b35113636ea310ac3cdcc076b2fb99003
Minor testing fix from libfritter

Change-Id: I3d5b4dcdbef4b5e7458d82541b78d8a1a50dcc98