prepare-gerrit-repos: Add Gitiles source submodule

Gitiles core can now be source-built from Gerrit's
modules/gitiles submodule.

Teach prepare-gerrit-repos to place a prepared Gitiles checkout
there, matching the existing handling for JGit and java-prettify.

Update the role tests so Gitiles is present in the prepared Gerrit
tree.

Change-Id: I9fc2871a8d809af9857d72ba0f80c81f4527dfe1