Preserve change creation time on project renames

* doc/source/gerrit.rst: When renaming projects in the changes
table, set created_on to itself so that it won't get updated to the
current time. This column is a timestamp datatype in MySQL and has
the "on update" property set, probably for no good reason but it's
how Gerrit's installer sets the schema.

Change-Id: I5b99d2869e55ab0f8b036fd08f248f406aff41bb
Merge "Preserve change creation time on project renames"