Fix imagebuilder to support `revision` attribute

A repo manifest can specify other branches on a project with the
`revision` attribute.  Use this value rather than the default value when
a project specifies it, and checking for git cleanliness of a project.

Change-Id: I41473f1728d042fcdee18b64377b338066d70eaf
(cherry picked from commit 1a0b79abf0f335f734319ba604b85f655ef08bfc)
2 files changed