Add the prep-source-repos project

This is a tool we've been using on the CI servers to let us use trunk +
unlanded patches, to enable us to take advantage of new
features/bugfixes before they have been fully reviewed.

This can be useful anywhere you are running code from a Gerrit repo but
have WIP or unreviewed patches you want to apply.

Change-Id: I8739847d67b653889bd854dbde94a82460409ad8
Merge "Add the prep-source-repos project"