git: Return errs.Error instead of stderr

The git package no longer returns stderr, but instead it wraps the error
and stderr being returned in an errs.Error

Change-Id: d3d2fd47f0
Story-Id: SF-79
wip [ci skip]

Change-Id: 1460f1319c
wip [ci skip]

Change-Id: 2cb10aee39
wip [ci skip]

Change-Id: a4a20ced77