gerrit/update-repos: Set up operations/mediawiki-config like production
We use a standard project.config for all repos imported into train-dev's Gerrit.
This commit adds the following additional project.config when importing the operations/mediawiki-config repo for the first time. Note that this change does not affect the repo if it already exists.
[submit] mergeContent = false action = rebase if necessary
This is the same configuration as production and is important for testing T343967.
Change-Id: If70b871939c31c06e7909c9122ed2011de30cc91